Allow send frames using monitor interface
when DFS chandef and we pass CAC (beaconing
allowed).

This fix problem when old kernel and new backports used,
in such case hostapd create/use also monitor interface.
Before this patch all frames hostapd send using monitor
iface were dropped when AP was configured on DFS channel.
Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>