can: populate netdev::dev_id for udev discrimination

My objective is to be able to totally discriminate CAN ports on multi-port
cards via udev so as to rename them to semantically interesting/unique names
for my system (e.g., "ecuCAN" and "auxCAN" instead of "can0" and "can1").

The following patch assigns the dev_id field to match the channel number on all
multi-channel devices. I can only test my two-port Peak PCI card, but it works
as expected: ATTRS{dev_id} now expresses the port number and my udev rules now
unambiguously pick out and rename my individual CAN ports.

Signed-off-by: Christopher R. Baker <cbaker@rec.ri.cmu.edu>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net> [PEAK PCAN-USB pro and EMS PCMCIA]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>