can: flexcan: increase FLEXCAN_MCR_MAXMB() macro to 7 bits

This patch increases the mask in the FLEXCAN_MCR_MAXMB() to 7 bits as in the
newer flexcan cores the MAXMB field is 7 bits wide.

Reported-by: David Jander <david@protonic.nl>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>