mac80211: make check_combinations() aware of chanctx reservation

The ieee80211_check_combinations() computes
radar_detect accordingly depending on chanctx
reservation status.

This makes it possible to use the function for
channel_switch validation.

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