net: phy: add device-managed devm_mdiobus_register

If there's no special ordering requirement for mdiobus_unregister(),
then driver code can be simplified by using a device-managed version
of mdiobus_register(). Prerequisite is that bus allocation has been
done device-managed too. Else mdiobus_free() may be called whilst
bus is still registered, resulting in a BUG_ON(). Therefore let
devm_mdiobus_register() return -EPERM if bus was allocated
non-managed.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>