mac80211: ignore quiet mode in probe

Some buggy APs keep the CSA IE in probes after the channel
switch was completed and can silence us for no good reason.
Apply quiet mode only from beacons. If there is real channel
switch going on, we will see the beacon anyway.

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>