cfg80211: correctly check ad-hoc channels

Ad-hoc requires beaconing for regulatory purposes. Validate that the
channel is valid for beaconing, and not only enabled.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>