[Ovmsdev] Extended CAN Frame support on CAN2/3

[Greg D.]

Hi folks,  (Mark, in particular),

I've tweaked the mcp2515.cpp code to support Extended CAN frames on CAN
2 & 3.  Needed this for the OBD2ECU code, but it's there for anyone else
who may want it.

Mark - I did a bit of this by trial & error, since some of what I was
doing wasn't working as I expected it would.  Bits upside down, that
sort of thing...  So I expect some of the middle bits around the three
frame flags is wrong, particularly on the transmit side, but for the
PIDs I use, the results I get match what Wireshark decodes.  So, I think
it's close.

Greg

p.s.  Got my first CAN 3 bus hang too...  hope it's not related. :)