batman-adv: make sure ELP/OGM orig MAC is updated on address change

When the MAC address of the primary interface is changed,
update the originator address in the ELP and OGM skb buffers as
well in order to reflect the change.

Fixes: d6f94d91

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