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/loopback.c b/drivers/net/loopback.c
index 51bbce7..1bc654a 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -69,7 +69,8 @@
  * The higher levels take care of making this non-reentrant (it's
  * called with bh's disabled).
  */
-static int loopback_xmit(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t loopback_xmit(struct sk_buff *skb,
+				 struct net_device *dev)
 {
 	struct pcpu_lstats *pcpu_lstats, *lb_stats;
 	int len;