mac80211: allow CSA to self with immediate quiet

Currently, due to some buggy APs that continue to include
CSA IEs after the switch, we ignore CSA to same channel.
However, some other APs may do CSA to self in order to have
immediate quiet. Allow it. Do it only for beacons.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>