Allow changing to DFS channels if the channel is available for
beaconing and userspace controls DFS operation.

Channel switch announcement from other stations on DFS channels will
be interpreted as radar event. These channels will then be marked as
unvailable.
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>