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/net/dm9000.c b/drivers/net/dm9000.c
index e402e91..dd771de 100644
--- a/drivers/net/dm9000.c
+++ b/drivers/net/dm9000.c
@@ -756,7 +756,7 @@
 	dm9000_dbg(db, 3, "%s:\n", __func__);
 
 	if (db->tx_pkt_cnt > 1)
-		return 1;
+		return NETDEV_TX_BUSY;
 
 	spin_lock_irqsave(&db->lock, flags);