mac80211: fix sdata->radar_required locking

radar_required setting wasn't protected by
local->mtx in some places. This should prevent
from scanning/radar detection/roc colliding.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
(cherry picked from http://mid.gmane.org/1390227670-19030-5-git-send-email-michal.kazior@tieto.com)