can: gw: Fix error path of cgw_module_init

This patch add error path for cgw_module_init to avoid possible crash if
some error occurs.

Fixes: c1aabdf3 ("can-gw: add netlink based CAN routing")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>