batman-adv: prepare for const netdev->dev_addr

netdev->dev_addr will be constant soon, make sure
the qualifier is propagated thru batman-adv.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>