ath9k: remove the WARN_ON that triggers if generating a beacon fails

During teardown, mac80211 will not return a new beacon. This is normal and
handled properly in the driver, so there's no need to spam the user with a kernel
warning here.

Cc: stable@vger.kernel.org
Signed-off-by: NFelix Fietkau <nbd@openwrt.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/beacon.c

@@ -360,7 +360,6 @@ void ath9k_beacon_tasklet(unsigned long data)
 		return;
 
 	bf = ath9k_beacon_generate(sc->hw, vif);
-	WARN_ON(!bf);
 
 	if (sc->beacon.bmisscnt != 0) {
 		ath_dbg(common, BSTUCK, "resume beacon xmit after %u misses\n",