can: gs_usb: check for kzalloc allocation failure

smatch detected the following issue:
drivers/net/can/usb/gs_usb.c:904 gs_usb_probe() error:
   potential null dereference 'dev'.  (kzalloc returns null)

Add a check for null return from kzalloc and return -ENOMEM

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>