mac80211: mesh: advertise gates in mesh formation

The Connected to Mesh Gate subfield (802.11-2016 9.4.2.98.7) in the Mesh
Formation Info field is currently unset.  This field may be useful in
determining which MBSSes to join or which mesh STAs to peer with.

If this mesh STA is a gate, by having turned on mesh gate announcements,
or if we have a path to one (e.g. by having received RANNs) then set this
bit to 1.

Signed-off-by: Bob Copeland <bobcopeland@fb.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>