mesh: Fix peer link counting when removing a mesh peer

mesh: Fix peer link counting when removing a mesh peer

When removing a mesh peer with control interface commands (ACCEPT_ACL
DEL_MAC/DENY_ACL ADD_MAC/MESH_PEER_REMOVE) the plink count was not
decreased from its last connection. This resulted in peer link count
leakage and wpa_supplicant rejecting the connections after reaching
max_peer_links (default: 99).

Fix this by decreasing the plink count when removing a mesh peer which
is in PLINK_ESTAB state.

Signed-off-by: Yu Wang <yyuwang@codeaurora.org>