ar9170: cancel led worker properly on exit

"[PATCH 3/4 v2] ar9170: fix LED power state handling" revealed
a bug which can cause a ugly crash.

The delayed worker is canceled before the LED class functions are
unregistered... So, if something manages to update the LEDs
while unregister routine is running the timer could fire _after_ the
module has been unloaded.
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ar9170/led.c

@@ -74,7 +74,7 @@ static void ar9170_update_leds(struct work_struct *work)
 
 	mutex_lock(&ar->mutex);
 	for (i = 0; i < AR9170_NUM_LEDS; i++)
-		if (ar->leds[i].toggled) {
+		if (ar->leds[i].registered && ar->leds[i].toggled) {
 			led_val |= 1 << i;
 
 			tmp = 70 + 200 / (ar->leds[i].toggled);
@@ -101,6 +101,9 @@ static void ar9170_led_brightness_set(struct led_classdev *led,
 	struct ar9170_led *arl = container_of(led, struct ar9170_led, l);
 	struct ar9170 *ar = arl->ar;
 
+	if (unlikely(!arl->registered))
+		return ;
+
 	if (arl->last_state != !!brightness) {
 		arl->toggled++;
 		arl->last_state = !!brightness;
@@ -139,13 +142,14 @@ void ar9170_unregister_leds(struct ar9170 *ar)
 {
 	int i;
 
-	cancel_delayed_work_sync(&ar->led_work);
-
 	for (i = 0; i < AR9170_NUM_LEDS; i++)
 		if (ar->leds[i].registered) {
 			led_classdev_unregister(&ar->leds[i].l);
 			ar->leds[i].registered = false;
+			ar->leds[i].toggled = 0;
 		}
+
+	cancel_delayed_work_sync(&ar->led_work);
 }
 
 int ar9170_register_leds(struct ar9170 *ar)