can: flexcan: Add check for transceiver maximum bitrate limitation

CAN FD can transmit up to 8Mbps, but some transceivers only can support
5Mbps, so add check in driver.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Link: https://lore.kernel.org/r/20191030064245.12923-2-qiangqing.zhang@nxp.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>