cfg80211: allow beaconing after CAC

After going throught the Channel Availability Check (CAC)
required by DFS enable beaconing. Channels that have gone
through a CAC will be in the NL80211_DFS_AVAILABLE.
Without this change APs don't start beaconing after
successful CAC.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
(cherry picked from http://mid.gmane.org/1383048716-5886-2-git-send-email-janusz.dziedzic@tieto.com)