ethernet: forcedeth: remove direct netdev->dev_addr writes

forcedeth writes to dev_addr byte by byte, make it use
a local buffer instead. Commit the changes with
eth_hw_addr_set() at the end.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>