sun8i_emac: Fix mdio read sequence

To send a parametrized command to the PHY over MDIO, we should write
the data first, the trigger the execution by the command register
write. Fix the access pattern in our MDIO write routine.
Apparently this doesn't really matter with the Realtek PHY on the
Pine64, but other PHYs (which require more setup) will choke on
the wrong order.
[Andre: add commit message]

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jagan Teki <jagan@openedev.com>