rh850: fix mtu to CANFD size

MTU defaults to standard CAN frame, however,
since rh850 driver supports CANFD frames MTU
needs to be set accordingly.

CRs-Fixed: 2071638
Change-Id: I5015af6e61ef4b4e021a715a8dec60d42a8a9e5b
Signed-off-by: Alex Yakavenka <ayakav@codeaurora.org>