ethernet: sun: remove direct netdev->dev_addr writes

Consify temporary variables pointing to netdev->dev_addr.

A few places need local storage but pretty simple conversion
over all. Note that macaddr[] is an array of ints, so we need
to keep the loops.

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