cfg80211: fix dfs channel state after stopping AP

This can be called when a mode of operation that can supports
initiating radiation on a DFS channel leaves that channel to
ensure that if the mode of operation is started again on it
that it will require a new channel availability check.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Reviewed-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
(cherry picked from http://mid.gmane.org/1382984824-3729-5-git-send-email-janusz.dziedzic@tieto.com)