mwl8k: Disable tx_wait completion

Whenever TX ring is drained, priv->tx_wait still points
to a valid completion. Making sure that it points to
NULL before returning from the  mwl8k_tx_wait_empty
check.
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwl8k.c

@@ -1588,6 +1588,7 @@ static int mwl8k_tx_wait_empty(struct ieee80211_hw *hw)
 
 		rc = -ETIMEDOUT;
 	}
+	priv->tx_wait = NULL;
 	spin_unlock_bh(&priv->tx_lock);
 
 	return rc;