6lowpan: handle dev_queue_xmit() error code properly

dev_queue_xmit() will return a positive value if the packet could not be
queued, often because the real network device (in our case the mac802154
wpan device) has its queue stopped.  lowpan_xmit() should handle the
positive return code (for the debug statement) and return that value to
the higher layer so the higher layer will retry sending the packet.
Signed-off-by: NAlan Ott <alan@signal11.us>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan.c

@@ -1139,10 +1139,10 @@ static netdev_tx_t lowpan_xmit(struct sk_buff *skb, struct net_device *dev)
 error:
 	dev_kfree_skb(skb);
 out:
-	if (err < 0)
+	if (err)
 		pr_debug("ERROR: xmit failed\n");
 
-	return (err < 0 ? NETDEV_TX_BUSY : NETDEV_TX_OK);
+	return (err < 0) ? NET_XMIT_DROP : err;
 }
 
 static struct wpan_phy *lowpan_get_phy(const struct net_device *dev)