Use CAN get/setPAyloadLength in kernel module

The kernel module in still does its own padding/length calculations in 1722ethernet.c, because the code predates the inline helpers. We should update that,using the "Known-good" functions from the CAN headers instead

Start in 1722ethernet.c and inspect the existing padding and length calculations alongside the CAN headers' get/setPayloadLength helpers. Replace the local calculations with those helpers and confirm the kernel module retains the expected payload lengths.

Written by the indexing model from the issue text.