net: mdio-mux: show errors on probe failure

Showing the precise error symbols can help debugging probe issues, such
as the recent -EIO error in of_mdiobus_register() caused by the lack of
bus->read_c45() and bus->write_c45() methods.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231213152712.320842-2-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>