can: j1939: replace fall through comment by fallthrough pseudo-keyword

Replace the existing /* fall through */ comments the new
pseudo-keyword macro fallthrough.

Cc: Robin van der Gracht <robin@protonic.nl>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20210616102811.2449426-3-mkl@pengutronix.de


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