ath9k: Fix kernel panic on module unload

The commit "ath9k: configure beacons based on hw opmode" introduced
a regression which leads to kernel panic. Failed to stop ani timer
during the driver unload while any of the beaconing vif is running.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/beacon.c b/drivers/net/wireless/ath/ath9k/beacon.c
index eccb0ec..b56f69e 100644
--- a/drivers/net/wireless/ath/ath9k/beacon.c
+++ b/drivers/net/wireless/ath/ath9k/beacon.c
@@ -320,6 +320,7 @@
 	if (avp->av_bcbuf != NULL) {
 		struct ath_buf *bf;
 
+		avp->is_bslot_active = false;
 		if (avp->av_bslot != -1) {
 			sc->beacon.bslot[avp->av_bslot] = NULL;
 			sc->nbcnvifs--;