net: pcs: xpcs: use mdiodev accessors

Use mdiodev accessors rather than accessing the bus and address in
the mdio_device structure and using the mdiobus accessors.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>