net: convert remaining non-symbolic return values in ndo_start_xmit() functions

This patch converts the remaining occurences of raw return values to their
symbolic counterparts in ndo_start_xmit() functions that were missed by the
previous automatic conversion.

Additionally code that assumed the symbolic value of NETDEV_TX_OK to be zero
is changed to explicitly use NETDEV_TX_OK.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/lib8390.c b/drivers/net/lib8390.c
index f28c233..d6be360 100644
--- a/drivers/net/lib8390.c
+++ b/drivers/net/lib8390.c
@@ -414,7 +414,7 @@
 	dev_kfree_skb (skb);
 	dev->stats.tx_bytes += send_length;
 
-	return 0;
+	return NETDEV_TX_OK;
 }
 
 /**