sh_eth: Simplify MDIO bus initialization and release

The network device passed to the sh_mdio_init and sh_mdio_release
functions is only used to access the sh_eth_private instance. Pass it
directly to those functions.

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>