can: gw: use struct canfd_frame as internal data structure

To prepare the CAN FD support this patch implements the first adaptions in
data structures for CAN FD without changing the current functionality.

Additionally some code at the end of this patch is moved or indented to
simplify the review of the next implementation step.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>