mac80211: Clear beacon_int in ieee80211_do_stop

This fixes stale beacon-int values that would keep a netdev
from going up.

To reproduce:

Create two VAP on one radio.
vap1 has beacon-int 100, start it.
vap2 has beacon-int 240, start it (and it will fail
  because beacon-int mismatch).
reconfigure vap2 to have beacon-int 100 and start it.
  It will fail because the stale beacon-int 240 will be used
  in the ifup path and hostapd never gets a chance to set the
  new beacon interval.

Cc: stable@vger.kernel.org
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -1015,6 +1015,8 @@ static void ieee80211_do_stop(struct ieee80211_sub_if_data *sdata,
 	if (local->open_count == 0)
 		ieee80211_clear_tx_pending(local);
 
+	sdata->vif.bss_conf.beacon_int = 0;
+
 	/*
 	 * If the interface goes down while suspended, presumably because
 	 * the device was unplugged and that happens before our resume,