of_mdio / mdiobus: ensure mdio devices have fwnode correctly populated

Ensure that all mdio devices populate the struct device fwnode pointer
as well as the of_node pointer to allow drivers that wish to use
fwnode APIs to work.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>