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

netdev->dev_addr is const now.

Maintain the questionable offsetting in ndo_set_mac_address.

Compile tested holly_defconfig and mpc7448_hpc2_defconfig.

Fixes: adeef3e3

 ("net: constify netdev->dev_addr")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>