mac80211: protect TDLS discovery session

After sending a TDLS discovery-request, we expect a reply to arrive on
the AP's channel. We must stay on the channel (no PSM, scan, etc.), since
a TDLS setup-response is a direct packet not buffered by the AP.
Add a new mac80211 driver callback to allow discovery session protection.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>