ethernet: prestera: use eth_hw_addr_gen()

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.

Vadym and Taras report that the current behavior of the driver
is not exactly expected and it's better to add the port id in
like other drivers do.

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