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/s390/net/netiucv.c b/drivers/s390/net/netiucv.c
index be716e4..aec9e5d 100644
--- a/drivers/s390/net/netiucv.c
+++ b/drivers/s390/net/netiucv.c
@@ -1315,9 +1315,9 @@
 		return NETDEV_TX_BUSY;
 	}
 	dev->trans_start = jiffies;
-	rc = netiucv_transmit_skb(privptr->conn, skb) != 0;
+	rc = netiucv_transmit_skb(privptr->conn, skb);
 	netiucv_clear_busy(dev);
-	return rc;
+	return rc ? NETDEV_TX_BUSY : NETDEV_TX_OK;
 }
 
 /**