ath9k: Prevent divide-by-zero upon bad beacon_interval.

A similar patch fixed crashes seen on an ath9k system
when testing against a broken ath10k AP.  This patch
is slightly less protective, but probably will do the
job and is less redundant.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/recv.c b/drivers/net/wireless/ath/ath9k/recv.c
index a01efd3..a1fac6b 100644
--- a/drivers/net/wireless/ath/ath9k/recv.c
+++ b/drivers/net/wireless/ath/ath9k/recv.c
@@ -538,8 +538,8 @@
 		sc->ps_flags &= ~PS_BEACON_SYNC;
 		ath_dbg(common, PS,
 			"Reconfigure beacon timers based on synchronized timestamp\n");
-		ath9k_set_beacon(sc);
-
+		if (!(WARN_ON_ONCE(sc->cur_beacon_conf.beacon_interval == 0)))
+			ath9k_set_beacon(sc);
 		if (sc->p2p_ps_vif)
 			ath9k_update_p2p_ps(sc, sc->p2p_ps_vif->vif);
 	}