Input: usbtouchscreen - actually check return value of usb_submit_urb()

Fixes the following W=1 kernel build warning(s):

 drivers/input/touchscreen/usbtouchscreen.c: In function ‘nexio_read_data’:
 drivers/input/touchscreen/usbtouchscreen.c:1052:50: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210114152323.2382283-4-lee.jones@linaro.org
[dtor: log error code as well]
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/usbtouchscreen.c

@@ -1044,6 +1044,7 @@ static void nexio_exit(struct usbtouch_usb *usbtouch)
 
 static int nexio_read_data(struct usbtouch_usb *usbtouch, unsigned char *pkt)
 {
+	struct device *dev = &usbtouch->interface->dev;
 	struct nexio_touch_packet *packet = (void *) pkt;
 	struct nexio_priv *priv = usbtouch->priv;
 	unsigned int data_len = be16_to_cpu(packet->data_len);
@@ -1062,6 +1063,8 @@ static int nexio_read_data(struct usbtouch_usb *usbtouch, unsigned char *pkt)
 
 	/* send ACK */
 	ret = usb_submit_urb(priv->ack, GFP_ATOMIC);
+	if (ret)
+		dev_warn(dev, "Failed to submit ACK URB: %d\n", ret);
 
 	if (!usbtouch->type->max_xc) {
 		usbtouch->type->max_xc = 2 * x_len;