net: um: use eth_hw_addr_set()

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 go through appropriate helpers.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>