batman-adv: turn batadv_neigh_node_get() into local function

commit c214ebe1eb29 ("batman-adv: move neigh_node list add into
batadv_neigh_node_new()") removed external calls to
batadv_neigh_node_get().

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