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/smsc9420.c b/drivers/net/smsc9420.c
index 60abdb1..514311d 100644
--- a/drivers/net/smsc9420.c
+++ b/drivers/net/smsc9420.c
@@ -968,7 +968,8 @@
 	}
 }
 
-static int smsc9420_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t smsc9420_hard_start_xmit(struct sk_buff *skb,
+					    struct net_device *dev)
 {
 	struct smsc9420_pdata *pd = netdev_priv(dev);
 	dma_addr_t mapping;