eth: platform: add a helper for loading 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. To maintain netdev->dev_addr in this tree we need to make all
the writes to it got through appropriate helpers.

There is a handful of drivers which pass netdev->dev_addr as
the destination buffer to eth_platform_get_mac_address().
Add a helper which takes a dev pointer instead, so it can call
an appropriate helper.

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