mac80211: fix recalc_radar hwconf sync problem

local->hw.conf maybe not be synced when recalcing whether radar is
enabled, sometimes leaving radar enabled even if it's not neccesary
anymore.

Fix this by:
 * setting radar_enabled when creating the chanctx
 * turning radar_enabled off before destroying the last channel context

Reported-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>