Liteeth does not transmit out UDP packets with length 16

[rowanG077]

I had some issues with transmitting out UDP packets with a 16 byte payload. After quite a bit of debugging my own code I found out it's actually Liteeth. in the process I fixed the timing issues with #151

But that was not the problem. So I made a small reproducer based on the colorlite project: https://github.com/rowanG077/liteeth-16-byte-bug

It does nothing more then instantiate a liteeth udp core and connect the rx to the tx with a packet fifo. It works flawlessly if you send any packet with length that is not 16 bytes.

[rowanG077]

I have at least been able to confirm the MAC layer is not the issue. I instantiated a MAC core loopback and send the same bytes. That works without issues.

[rowanG077]

Did the same tests using the IP layer. Works as well. This is something that's caused by the UDP core somehow.

[rowanG077]

I lied actually. I did a full sweep using the project I linked from 1 byte UDP payload to 1458 payload. 15 byte and 16 byte payloads are the only problematic ones.