When a mesh point reconnects by starting from Authentication frame
sequence, the plink count was not decremented from its last connection.
This resulted in leaking peer link count and causing wpa_supplicant to
reject the connection after max_peer_links (default: 99) reconnects.
This was reproduced by pre-configuring 2 mesh points with mesh
credentials. Boot both mesh points and make sure they connect to each
other. Then in a loop reboot one of the mesh points after it
successfully connects while leaving the other mesh point up and running.
After 99 iterations the supplicant on mesh point that is not rebooting
will reject the connection request from the other mesh point.
Fix this by decrementing num_plinks when freeing a STA entry that is
still in PLINK_ESTAB state.
Signed-off-by: Srinivasa Duvvuri <sduvvuri@chromium.org>