net: phylink: Convert to mdiobus_c45_{read|write}

Stop using the helpers to construct a special phy address which
indicates C45. Instead use the C45 accessors, which will call the
busses C45 specific read/write API.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>