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/hamradio/baycom_epp.c

@@ -777,7 +777,7 @@ static int baycom_send_packet(struct sk_buff *skb, struct net_device *dev)
 		return 0;
 	}
 	if (bc->skb)
-		return -1;
+		return NETDEV_TX_LOCKED;
 	/* strip KISS byte */
 	if (skb->len >= HDLCDRV_MAXFLEN+1 || skb->len < 3) {
 		dev_kfree_skb(skb);