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/cassini.c b/drivers/net/cassini.c
index 299a33b..7517dc1 100644
--- a/drivers/net/cassini.c
+++ b/drivers/net/cassini.c
@@ -2918,7 +2918,7 @@
 	return 0;
 }
 
-static int cas_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t cas_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct cas *cp = netdev_priv(dev);