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>

drivers/net/skfp/skfddi.c

@@ -1082,7 +1082,7 @@ static int skfp_send_pkt(struct sk_buff *skb, struct net_device *dev)
 	if (bp->QueueSkb == 0) {	// return with tbusy set: queue full
 
 		netif_stop_queue(dev);
-		return 1;
+		return NETDEV_TX_BUSY;
 	}
 	bp->QueueSkb--;
 	skb_queue_tail(&bp->SendSkbQueue, skb);