batman-adv: initialize ELP orig address on secondary interfaces

This fix prevents nodes to wrongly create a 00:00:00:00:00:00 originator
which can potentially interfere with the rest of the neighbor statistics.

Fixes: d6f94d91 ("batman-adv: ELP - adding basic infrastructure")
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>