net: constify netdev->dev_addr

Commit 406f42fa ("net-next: When a bond have a massive amount
of VLANs...") introduced a rbtree for faster Ethernet address look
up. We converted all users to make modifications via appropriate
helpers, make netdev->dev_addr const.

The update helpers need to upcast from the buffer to
struct netdev_hw_addr.

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