sh_eth: Use the platform device for memory allocation

Memory allocated for the MDIO bus with the devm_kzalloc() API is
associated with the network device. While this will cause memory to be
freed at the right time, it doesn't allow allocating memory before the
network device is initialized.

Replace the network device with the parent platform device for memory
allocation to remove that dependency. This also improves consistency
with the other devm_* calls in the driver that all use 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>