iwlwifi: return NETDEV_TX_OK from _tx ops

be consistent with mac80211 drivers and return correct return code.
NETDEV_TX_OK is 0, but we need to be consistent wrt formatting amongst
implementations

re: http://marc.info/?l=linux-wireless&m=123119327419865&w=2

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-agn.c

@@ -2482,7 +2482,7 @@ static int iwl_mac_tx(struct ieee80211_hw *hw, struct sk_buff *skb)
 		dev_kfree_skb_any(skb);
 
 	IWL_DEBUG_MACDUMP("leave\n");
-	return 0;
+	return NETDEV_TX_OK;
 }
 
 static int iwl_mac_add_interface(struct ieee80211_hw *hw,

drivers/net/wireless/iwlwifi/iwl3945-base.c

@@ -6538,7 +6538,7 @@ static int iwl3945_mac_tx(struct ieee80211_hw *hw, struct sk_buff *skb)
 		dev_kfree_skb_any(skb);
 
 	IWL_DEBUG_MAC80211("leave\n");
-	return 0;
+	return NETDEV_TX_OK;
 }
 
 static int iwl3945_mac_add_interface(struct ieee80211_hw *hw,