Most functions were using asserts, but in some cases it might not be a
programmatic error to pass an invalid ifnum. This makes sure the code
does not crash by testing it at runtim and returning an error.
The CAN stack support only raw CAN at this time.
It contains a device interface (drivers/include/can/candev.h) and the data link
layer, build around can/device.c can/pkt.c and can/router.c. can/dll.c contains
the upper level and lower level interface to send and receive CAN frames.
Upper layer interface is located in include/can/raw.h
