net: mdiobus: get rid of a BUG_ON()

We spotted a bug recently during a review where a driver was
unregistering a bus that wasn't registered, which would trigger this
BUG_ON().  Let's handle that situation more gracefully, and just print
a warning and return.
Reported-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio_bus.c

@@ -607,7 +607,8 @@ void mdiobus_unregister(struct mii_bus *bus)
 	struct mdio_device *mdiodev;
 	int i;
 
-	BUG_ON(bus->state != MDIOBUS_REGISTERED);
+	if (WARN_ON_ONCE(bus->state != MDIOBUS_REGISTERED))
+		return;
 	bus->state = MDIOBUS_UNREGISTERED;
 
 	for (i = 0; i < PHY_MAX_ADDR; i++) {