cfg80211: allow beaconing after DFS CAC

Allow beconing after we pass Channel Availability Check (CAC).
Allow non-DFS and DFS channels mix. All DFS channels have to
be in NL80211_DFS_AVAILABLE state (pass CAC).

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