Staging: batman-adv: Introduce update_primary_addr to update mac address

set_primary_if is currently misused to update the mac address in vis
packets. This unneeded and introduces overhead due to other operations
which must be done when updating the primary interface.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>