net: dsa: realtek: migrate user_mii_bus setup to realtek_dsa

In the user MDIO driver, despite numerous references to SMI, including
its compatible string, there's nothing inherently specific about the SMI
interface in the user MDIO bus. Consequently, the code has been migrated
to the rtl83xx module. All references to SMI have been eliminated.

The MDIO bus id was changed from Realtek-<switch id> to the switch
devname suffixed with :user_mii, giving more information about the bus
it is referencing.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>