net: use symbolic values for ndo_start_xmit() return codes

Convert magic values 1 and -1 to NETDEV_TX_BUSY and NETDEV_TX_LOCKED respectively.

0 (NETDEV_TX_OK) is not changed to keep the noise down, except in very few cases
where its in direct proximity to one of the other values.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/usb/gadget/u_ether.c b/drivers/usb/gadget/u_ether.c
index 4007770..016f63b 100644
--- a/drivers/usb/gadget/u_ether.c
+++ b/drivers/usb/gadget/u_ether.c
@@ -520,7 +520,7 @@
 	 */
 	if (list_empty(&dev->tx_reqs)) {
 		spin_unlock_irqrestore(&dev->req_lock, flags);
-		return 1;
+		return NETDEV_TX_BUSY;
 	}
 
 	req = container_of(dev->tx_reqs.next, struct usb_request, list);