net: dsa: add missing dsa_switch mdiobus remove

To prevent memory leakage on unbinding, add missing mdiobus unregister
and unallocation calls.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
index ed9d43f..14fac4e 100644
--- a/net/dsa/dsa.c
+++ b/net/dsa/dsa.c
@@ -424,6 +424,8 @@
 	if (ds->hwmon_dev)
 		hwmon_device_unregister(ds->hwmon_dev);
 #endif
+	mdiobus_unregister(ds->slave_mii_bus);
+	mdiobus_free(ds->slave_mii_bus);
 }
 
 #ifdef CONFIG_PM_SLEEP