mmcsd: fix a logic error

[gneworld]

Summary

When attempting to assign a value greater than 1 to a 1-bit bit-field, the result is undefined.
On many systems, this may cause the value to be truncated, keeping only the least significant bit.
In this specific case, the result of scr[0] & 2 is 2 (binary 10), but only the least significant bit (0) will be stored

Impact

mmcsd

Testing

NA

[lupyuen]

[Experimental Bot, please feedback here]

The provided PR summary does not meet the NuttX requirements. Here's why and what's missing:

What's Missing & How to Improve:

• Insufficient Summary:

  • Missing "Why": The summary states the technical problem but doesn't explain why this change is necessary. Is this a bug fix? Does it improve performance or code clarity?
  • Missing Scope: What functional part of the code is affected (e.g., SD card driver, networking stack, etc.)? Be specific.
  • Missing "How": The summary mentions truncation but doesn't clearly explain how the code is being changed to fix the issue. Provide a concise explanation of the solution.
  • Missing Links: Are there related NuttX issues or pull requests? Providing links is crucial for context and tracking.

• Incomplete Impact Assessment:

  • "mmcsd" is too vague. Specify the impact on users of the mmcsd driver. Will their applications need any changes? Will this fix observable issues?
  • Address all impact categories: Even if the impact is "NO," state it explicitly for each category (build, hardware, documentation, security, compatibility). If the answer is "YES," provide details.

• Testing Section is Inadequate:

  • "NA" is not acceptable. You must provide testing information.
  • Details, Details, Details: Include the following:
    • Build Host(s): Operating system, CPU architecture, compiler (name and version) used to build NuttX.
    • Target(s): The specific hardware platform or simulator you tested on. Include the board name and configuration.
    • Testing Logs (Before & After): Relevant log snippets demonstrating the issue before the change and the fix after the change. This provides concrete evidence that your PR works.

Example of an Improved Summary:

## Summary

This PR fixes a bug in the SD/MMC driver (mmcsd) where assigning a value greater than 1 to a 1-bit bit-field led to undefined behavior. This could cause unexpected card initialization failures or data corruption. 

The issue is resolved by ensuring that only valid values (0 or 1) are written to the affected bit-field. 

**Related Issue:** [Link to NuttX issue if applicable] 

## Impact

* **Is new feature added?** NO
* **Is existing feature changed?** YES - The SD/MMC driver (mmcsd) is fixed to handle bit-field assignments correctly.
* **Impact on user:**  Users relying on the mmcsd driver may see improved reliability, especially during SD card initialization. Applications themselves should not require changes.
* **Impact on build:** NO
* **Impact on hardware:** NO
* **Impact on documentation:** YES - The mmcsd driver documentation should be updated to mention this fix and the correct usage of the affected bit-field. Documentation updates will be provided in a separate commit.
* **Impact on security:**  Potentially YES - The previous undefined behavior could have led to security vulnerabilities, though none are known at this time.
* **Impact on compatibility:** NO 
* **Anything else to consider?**  N/A

## Testing

**Build Host:**
* OS: Ubuntu 20.04
* CPU: x86_64 
* Compiler: GCC 9.4.0

**Target:**
* Arch: ARM 
* Board: STM32F4 Discovery (stm32f4discovery)

**Testing Logs Before Change:**

[logs showing the issue before the fix, e.g., SD card initialization failure]

**Testing Logs After Change:**

[logs showing successful SD card initialization and operation]

Remember: Tailor this example to your specific PR, providing accurate and detailed information.

[pkarashchenko]

LGTM!