can: introduce CAN_REQUIRED_SIZE macro

The size of this structure will be increased with J1939 support. To stay
binary compatible, the CAN_REQUIRED_SIZE macro is introduced for
existing CAN protocols.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>