net: dsa: ar9331: register the mdiobus under devres

As explained in commits:
74b6d7d1 ("net: dsa: realtek: register the MDIO bus under devres")
5135e96a ("net: dsa: don't allocate the slave_mii_bus using devres")

mdiobus_free() will panic when called from devm_mdiobus_free() <-
devres_release_all() <- __device_release_driver(), and that mdiobus was
not previously unregistered.

The ar9331 is an MDIO device, so the initial set of constraints that I
thought would cause this (I2C or SPI buses which call ->remove on
->shutdown) do not apply. But there is one more which applies here.

If the DSA master itself is on a bus that calls ->remove from ->shutdown
(like dpaa2-eth, which is on the fsl-mc bus), there is a device link
between the switch and the DSA master, and device_links_unbind_consumers()
will unbind the ar9331 switch driver on shutdown.

So the same treatment must be applied to all DSA switch drivers, which
is: either use devres for both the mdiobus allocation and registration,
or don't use devres at all.

The ar9331 driver doesn't have a complex code structure for mdiobus
removal, so just replace of_mdiobus_register with the devres variant in
order to be all-devres and ensure that we don't free a still-registered
bus.

Fixes: ac3a68d5 ("net: phy: don't abuse devres in devm_mdiobus_register()")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/qca/ar9331.c

@@ -378,7 +378,7 @@ static int ar9331_sw_mbus_init(struct ar9331_sw_priv *priv)
 	if (!mnp)
 		return -ENODEV;
 
-	ret = of_mdiobus_register(mbus, mnp);
+	ret = devm_of_mdiobus_register(dev, mbus, mnp);
 	of_node_put(mnp);
 	if (ret)
 		return ret;
@@ -1091,7 +1091,6 @@ static void ar9331_sw_remove(struct mdio_device *mdiodev)
 	}
 
 	irq_domain_remove(priv->irqdomain);
-	mdiobus_unregister(priv->mbus);
 	dsa_unregister_switch(&priv->ds);
 
 	reset_control_assert(priv->sw_reset);