net: dsa: realtek: common rtl83xx module

Some code can be shared between both interface modules (MDIO and SMI)
and among variants. These interface functions migrated to a common
module:

- rtl83xx_lock
- rtl83xx_unlock
- rtl83xx_probe
- rtl83xx_register_switch
- rtl83xx_unregister_switch
- rtl83xx_shutdown
- rtl83xx_remove

The reset during probe was moved to the end of the common probe. This way,
we avoid a reset if anything else fails.

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>