net: Rework mdio-ofgpio driver to use of_mdio infrastructure

Changes to the fs_enet driver aa73832c
("net: Rework fs_enet driver to use of_mdio infrastructure")
cause kernel crashes when using the mdio-ofgpio driver.
 
This patch replicates similar changes made to the fs_enet mii-bitbang
drivers.  It has been tested on a custom mpc8280 based board using an
NFS mounted root.
 
Signed-off-by: Mark Ware <mware@elphinstone.net>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>