batman-adv: Provide teardown hook for batadv_vlan proto

The batadv_vlan proto doesn't need to do anything when it gets teared down.
But the scripts are still trying to call the teardown function of this
proto. This results in warnings like:

    daemon.notice netifd: batmesh1 (18940): ./batadv_vlan.sh: eval: line 37: proto_batadv_vlan_teardown: not found

Just providing a stub function avoids this log spam.

Fixes: #1044
Reported-by: Rani Hod <rani.hod@gmail.com>
Fixes: f5205d7d

 ("batman-adv: upgrade package to latest release 2014.2.0")
Signed-off-by: Sven Eckelmann <sven@narfation.org>