mac80211: mesh_plink: handle confirm frames with new plid

The 802.11 standard says when processing a plink confirm
frame:

"If the peerLinkID in the mesh peering instance has not been
set, the Local Link ID field of the Mesh Peering Confirm
request shall be copied into the peerLinkID in the mesh
peering instance."

We were only doing this when receiving an open peering frame,
but it could happen that the open frame gets lost and so we
should handle this case rather than rejecting the confirm and
failing the whole peering process.

Reported-by: Yu Niiro <yu.niiro@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>