[Ovmsdev] Extended CAN Frame support on CAN2/3

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. :)