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

drivers/net/can/usb/gs_usb.c

@@ -901,6 +901,8 @@ static int gs_usb_probe(struct usb_interface *intf, const struct usb_device_id *
 	}
 
 	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
+	if (!dev)
+		return -ENOMEM;
 	init_usb_anchor(&dev->rx_submitted);
 
 	atomic_set(&dev->active_channels, 0);