netdev: convert bulk of drivers to netdev_tx_t

In a couple of cases collapse some extra code like:
   int retval = NETDEV_TX_OK;
   ...
   return retval;
into
   return NETDEV_TX_OK;

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/8139cp.c b/drivers/net/8139cp.c
index 4a8995a..462d9f59 100644
--- a/drivers/net/8139cp.c
+++ b/drivers/net/8139cp.c
@@ -736,7 +736,8 @@
 		netif_wake_queue(cp->dev);
 }
 
-static int cp_start_xmit (struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t cp_start_xmit (struct sk_buff *skb,
+					struct net_device *dev)
 {
 	struct cp_private *cp = netdev_priv(dev);
 	unsigned entry;