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/ifb.c b/drivers/net/ifb.c
index 96713ef..0a79b45 100644
--- a/drivers/net/ifb.c
+++ b/drivers/net/ifb.c
@@ -164,7 +164,7 @@
 {
 	struct ifb_private *dp = netdev_priv(dev);
 	struct net_device_stats *stats = &dev->stats;
-	int ret = 0;
+	int ret = NETDEV_TX_OK;
 	u32 from = G_TC_FROM(skb->tc_verd);
 
 	stats->rx_packets++;