net: mdio: of_mdiobus_register(): fall back to mdiobus_register() for !CONFIG_OF

If CONFIG_OF is not set, make of_mdiobus_register() call
mdiobus_register() instead of returning -ENOSYS.

This way, we can just call of_mdiobus_register() from all DT-enabled
drivers to handle the compat cases.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>