carl9170: Fix mismatch in carl9170_op_set_key mutex lock-unlock

If is_main_vif(ar, vif) reports that we have to fall back
to software encryption, we goto err_softw; before locking ar->mutex.
As a result, we have unprotected call to carl9170_set_operating_mode
and unmatched mutex_unlock.

The patch fix the issue by adding mutex_lock before goto.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: <stable@kernel.org>
Acked-By: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/carl9170/main.c

@@ -1066,8 +1066,10 @@ static int carl9170_op_set_key(struct ieee80211_hw *hw, enum set_key_cmd cmd,
 	 * the high througput speed in 802.11n networks.
 	 */
 
-	if (!is_main_vif(ar, vif))
+	if (!is_main_vif(ar, vif)) {
+		mutex_lock(&ar->mutex);
 		goto err_softw;
+	}
 
 	/*
 	 * While the hardware supports *catch-all* key, for offloading