wl12xx: Fix kernel crash related to hw recovery and interface shutdown

It is possible that the op_remove_interface function  is invoked exactly at
the same time has hw recovery is started. In this case it is possible for the
interface to be already removed in the op_remove_interface call, which
currently leads to a kernel warning and a subsequent kernel crash.

Fix this by ignoring the op_remove_interface call if the interface is already
down at that point.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Tested-by: Tuomas Katila <ext-tuomas.2.katila@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>

drivers/net/wireless/wl12xx/main.c

@@ -1157,10 +1157,16 @@ static void wl1271_op_remove_interface(struct ieee80211_hw *hw,
 	struct wl1271 *wl = hw->priv;
 
 	mutex_lock(&wl->mutex);
-	WARN_ON(wl->vif != vif);
-	__wl1271_op_remove_interface(wl);
-	mutex_unlock(&wl->mutex);
+	/*
+	 * wl->vif can be null here if someone shuts down the interface
+	 * just when hardware recovery has been started.
+	 */
+	if (wl->vif) {
+		WARN_ON(wl->vif != vif);
+		__wl1271_op_remove_interface(wl);
+	}
 
+	mutex_unlock(&wl->mutex);
 	cancel_work_sync(&wl->recovery_work);
 }