ath9k: Fix a locking bug in ath9k_add_interface()

We tried to revert commit d9c52fd ("ath9k: fix tx99 with monitor
mode interface") but accidentally missed part of the locking change.

The lock has to be held earlier so that we're holding it when we do
"sc->tx99_vif = vif;" and also there in the current code there is a
stray unlock before we have taken the lock.

Fixes: 6df0580 ("ath9k: add back support for using active monitor interfaces for tx99")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath9k/main.c

@@ -1251,6 +1251,7 @@ static int ath9k_add_interface(struct ieee80211_hw *hw,
 	struct ath_vif *avp = (void *)vif->drv_priv;
 	struct ath_node *an = &avp->mcast_node;
 
+	mutex_lock(&sc->mutex);
 	if (IS_ENABLED(CONFIG_ATH9K_TX99)) {
 		if (sc->cur_chan->nvifs >= 1) {
 			mutex_unlock(&sc->mutex);
@@ -1259,8 +1260,6 @@ static int ath9k_add_interface(struct ieee80211_hw *hw,
 		sc->tx99_vif = vif;
 	}
 
-	mutex_lock(&sc->mutex);
-
 	ath_dbg(common, CONFIG, "Attach a VIF of type: %d\n", vif->type);
 	sc->cur_chan->nvifs++;