cfg80211: fix start_radar_detection issue

After patch:
cfg80211/mac80211: refactor cfg80211_chandef_dfs_required()

start_radar_detection always fail with -EINVAL.

Acked-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>