at76c50x-usb: fix warning caused by at76_mac80211_tx now returning void

  CC [M]  drivers/net/wireless/at76c50x-usb.o
drivers/net/wireless/at76c50x-usb.c: In function ‘at76_mac80211_tx’:
drivers/net/wireless/at76c50x-usb.c:1759:4: warning: ‘return’ with a value, in function returning void

This is fallout from commit 7bb45683
("mac80211: make tx() operation return void").
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/at76c50x-usb.c

@@ -1756,7 +1756,8 @@ static void at76_mac80211_tx(struct ieee80211_hw *hw, struct sk_buff *skb)
 		if (compare_ether_addr(priv->bssid, mgmt->bssid)) {
 			memcpy(priv->bssid, mgmt->bssid, ETH_ALEN);
 			ieee80211_queue_work(hw, &priv->work_join_bssid);
-			return NETDEV_TX_BUSY;
+			dev_kfree_skb_any(skb);
+			return;
 		}
 	}