sh_eth: Use the platform device as the MDIO bus parent

The MDIO bus parent is set to the network device. Beside not reflecting
the hardware topology, this prevents registering the MDIO bus before
initializing the network device. Fix it by setting the MDIO bus parent
to the platform device.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>