can: sja1000: plx_pci: add support for ASEM CAN raw device

This patch adds support for ASEM opto-isolated dual channels
CAN raw device (http://www.asem.it)

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>