of_mdio: mdio_device_create() never returns NULL

mdio_device_create() never returns NULL, thus checking for it in
of_mdiobus_register_device() is pointless...

Suggested-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>