batman-adv: fix vlan header access

When batadv_get_vid() is invoked in interface_rx() the
batman-adv header has already been removed, therefore
the header_len argument has to be 0.

Introduced by c018ad3d
("batman-adv: add the VLAN ID attribute to the TT entry")

Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>