can: fix SJA1000 dlc for RTR packets

RTR frames do have a valid data length code on CAN.
The driver for SJA1000 did not handle that situation properly.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>