drivers/mge-hid.c: refactor, treat ABM unknown as disabled

[desertwitch]

I tried to illustrate here my code review/comments on the PR: networkupstools#2660

1. We've seen non-ABM capable devices with partial ABM paths, these were previously sent into ABM parsing functions. Treating ABM_UNKNOWN as ABM_DISABLED resolves this problem, ABM parsing functions should only be executed if ABM is explicitly ABM_ENABLED. This also allows for graceful transition from ABM_UNKNOWN/ABM_DISABLED to ABM_ENABLED.

2. Removed ABM_ENABLED_TYPE as an additional way of enabling ABM, this is just a textual value for the battery charger type and ABM disabled/enabled is already handled through the correct HID path (x.ABMEnable) in another function. An ABM battery charger type can still be ABM disabled, so it makes no sense to assume enable ABM based on the charger type.

3. Improved the two ABM decision paths (old + new) with clear structure, comments, improved function and variable names.

4. Improved the checking of ABM enabled-ness by checking for known values, rather than assigning any values directly.

5. Improved debug messages to be more clear and condensed, as they seemed to be partially overlapping in some areas.

6. Improved multiple implicitly casted variables to explicit casting in a single place where it felt correct to do so.