can: af_can: Fix error path of can_init()

This patch add error path for can_init() to avoid possible crash if some
error occurs.

Fixes: 0d66548a

 ("[CAN]: Add PF_CAN core module")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>