net: mdio_bus: add refcounting for fwnodes to mdiobus

Luiz Angelo Daros de Luca reports that the MDIO bus code maintains a
reference to the DT node, but does not hold a refcount on the node.

The simple solution to this is to add the necessary refcounting into
the MDIO bus code for all users, ensuring that on registration, the
refcount is incremented, and only dropped when the MDIO bus is
released.

Do this for fwnodes, so we not only fix this for DT, but also other
types of firmware nodes as well.

Reported-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>