vehicle.average.speed CAN signal currently not useful

Commit 006598e in low-can's master branch set message 3D9 containing the vehicle.average.speed signal definition as a CAN FD message, where it had previously been a regular, non-FD one.  This makes it not particularly usable on the AGL development platforms, as none of them are CAN FD capable AFAICT.  As well, this makes driving things like the dashboard app via an attached CAN device non-workable for the planned CES demo.  Based on discussion on the connectivity call on 11/28, I believe there was consensus that it should be reverted back to non-FD, with a follow-on change to define a separate CAN FD message to allow testing with vcan.