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

drivers/net/mdio/mdio-mux.c

@@ -190,8 +190,8 @@ int mdio_mux_init(struct device *dev,
 		r = of_property_read_u32(child_bus_node, "reg", &v);
 		if (r) {
 			dev_err(dev,
-				"Error: Failed to find reg for child %pOF\n",
-				child_bus_node);
+				"Error: Failed to find reg for child %pOF: %pe\n",
+				child_bus_node, ERR_PTR(r));
 			continue;
 		}
 
@@ -229,8 +229,8 @@ int mdio_mux_init(struct device *dev,
 			}
 			devm_kfree(dev, cb);
 			dev_err(dev,
-				"Error: Failed to register MDIO bus for child %pOF\n",
-				child_bus_node);
+				"Error: Failed to register MDIO bus for child %pOF: %pe\n",
+				child_bus_node, ERR_PTR(r));
 		} else {
 			cb->next = pb->children;
 			pb->children = cb;