net: phy: mdio-mux: slience probe defer error

If we fail to register the mdio bus due to probe defer, we should not
print an error message. Just be silent in this case.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>