can: dev: add can_is_canfd_skb() API

The CAN device drivers can use can_is_canfd_skb() to check if the frame to send
is on CAN FD mode or normal CAN mode.

Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Dong Aisheng <b29396@freescale.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>