can: ucan: fix non-atomic allocation in completion handler

USB completion handlers are called in atomic context and must
specifically not allocate memory using GFP_KERNEL.

Fixes: 9f2d3ea ("can: ucan: add driver for Theobroma Systems UCAN devices")
Cc: stable <stable@vger.kernel.org>     # 4.19
Cc: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Cc: Martin Elshuber <martin.elshuber@theobroma-systems.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/ucan.c

@@ -792,7 +792,7 @@ static void ucan_read_bulk_callback(struct urb *urb)
 			  up);
 
 	usb_anchor_urb(urb, &up->rx_urbs);
-	ret = usb_submit_urb(urb, GFP_KERNEL);
+	ret = usb_submit_urb(urb, GFP_ATOMIC);
 
 	if (ret < 0) {
 		netdev_err(up->netdev,