mdio: Abstract device_remove() and device_free()

Make device_free and device_remove operations in the mdio device
structure, so the core code does not need to differentiate between
phy devices and generic mdio devices.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>