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>