can: flexcan: Add provision for variable payload size

Till now the flexcan module supported 8 byte payload size as per CAN 2.0
specifications. But now upcoming flexcan module in NXP LX2160A SOC
supports CAN FD protocol too. The Message buffers need to be configured
to have payload size 64 bytes.

Therefore, added provision in the driver for payload size to be 64 bytes.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>