cfg80211: remove racy beacon_interval assignment

In case of AP mode, the beacon interval is already reset to
zero inside cfg80211_stop_ap(), and in the other modes it
isn't relevant. Remove the assignment to remove a potential
race since the assignment isn't properly locked.

Reported-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>