mac80211: make sure TDLS peer STA exists during setup

Make sure userspace added a TDLS peer station before invoking the
transmission of the first setup frame. This ensures packets to the peer
won't go through the AP path.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>