net: mdio-mux: add mdio_mux parameter to mdio_mux_init()

mdio_mux_init() use the parameter dev for two distinct thing:
1) Have a device for all devm_ functions
2) Get device_node from it

Since it is two distinct purpose, this patch add a parameter mdio_mux
that is linked to task 2.

This will also permit to register an of_node mdio-mux that lacks a direct
owning device.
For example a mdio-mux which is a subnode of a real device.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>