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/tulip/de2104x.c

@@ -612,7 +612,7 @@ static int de_start_xmit (struct sk_buff *skb, struct net_device *dev)
 	if (tx_free == 0) {
 		netif_stop_queue(dev);
 		spin_unlock_irq(&de->lock);
-		return 1;
+		return NETDEV_TX_BUSY;
 	}
 	tx_free--;