net: ax88796c: don't write to netdev->dev_addr directly

The future is here, convert the new driver as we are about
to make netdev->dev_addr const.

Acked-by: Lukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>