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/sc92031.c b/drivers/net/sc92031.c
index e3156c9..8d60300 100644
--- a/drivers/net/sc92031.c
+++ b/drivers/net/sc92031.c
@@ -941,7 +941,8 @@
 	return &dev->stats;
 }
 
-static int sc92031_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t sc92031_start_xmit(struct sk_buff *skb,
+				      struct net_device *dev)
 {
 	struct sc92031_priv *priv = netdev_priv(dev);
 	void __iomem *port_base = priv->port_base;