ethernet: i825xx: don't write directly to netdev->dev_addr

netdev->dev_addr is const now.

Compile tested rpc_defconfig w/ GCC 8.5.

Fixes: adeef3e3

 ("net: constify netdev->dev_addr")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>