can: flexcan: use struct canfd_frame for CAN classic frame

This patch prepares for CAN FD mode, using struct canfd_frame can both
for classic format frame and fd format frame.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Link: https://lore.kernel.org/r/20190712075926.7357-3-qiangqing.zhang@nxp.com


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