can: af_can: can_rx_register(): use max() instead of open coding it

This patch replaces an open coded max by the proper kernel define max().

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