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/s2io.c b/drivers/net/s2io.c
index 3138df5..ddccf5f 100644
--- a/drivers/net/s2io.c
+++ b/drivers/net/s2io.c
@@ -4072,7 +4072,7 @@
  *  0 on success & 1 on failure.
  */
 
-static int s2io_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t s2io_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct s2io_nic *sp = netdev_priv(dev);
 	u16 frg_cnt, frg_len, i, queue, queue_len, put_off, get_off;