net: dsa: qca8k: skip MDIO bus creation if its OF node has status = "disabled"

Currently the driver calls the non-OF devm_mdiobus_register() rather
than devm_of_mdiobus_register() for this case, but it seems to rather
be a confusing coincidence, and not a real use case that needs to be
supported.

If the device tree says status = "disabled" for the MDIO bus, we
shouldn't need an MDIO bus at all. Instead, just exit as early as
possible and do not call any MDIO API.

Signed-off-by: Vladimir Oltean <[e-mail]>
Reviewed-by: Alvin Šipraga <[e-mail]>
Reviewed-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/qca/qca8k-8xxx.c

···
 	struct dsa_switch *ds = priv->ds;
 	struct device_node *mdio;
 	struct mii_bus *bus;
-	int err;
+	int err = 0;
 
 	mdio = of_get_child_by_name(priv->dev->of_node, "mdio");
+	if (mdio && !of_device_is_available(mdio))
+		goto out;
 
 	bus = devm_mdiobus_alloc(ds->dev);
 	if (!bus) {
···
 	ds->user_mii_bus = bus;
 
 	/* Check if the devicetree declare the port:phy mapping */
-	if (of_device_is_available(mdio)) {
+	if (mdio) {
 		bus->name = "qca8k user mii";
 		bus->read = qca8k_internal_mdio_read;
 		bus->write = qca8k_internal_mdio_write;
···
 
 out_put_node:
 	of_node_put(mdio);
-
+out:
 	return err;
 }