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/niu.c b/drivers/net/niu.c
index 3ada7ea..119fd4e 100644
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -6657,7 +6657,8 @@
 	return ret;
 }
 
-static int niu_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t niu_start_xmit(struct sk_buff *skb,
+				  struct net_device *dev)
 {
 	struct niu *np = netdev_priv(dev);
 	unsigned long align, headroom;