batman-adv: Merge batadv_v_ogm_orig_update into batadv_v_ogm_route_update

Since batadv_v_ogm_orig_update() was only called from one place and the
calling function became very short, merge these two functions together.

This should also reflect the protocol description of B.A.T.M.A.N. V
better.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>