can: sja1000: Add support for CAN_CTRLMODE_LOOPBACK

This adds support for hardware loopback in SJA1000 by utilising its self
reception request (SRR) feature. Upon SRR the message is transmitted and
received simultaneously, meaning you can't have hardware loopback
without actually sending a message to the CAN bus in case of SJA1000.

Signed-off-by: Nikita Edward Baruzdin <nebaruzdin@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>