net: phy: print stack trace in phy_error

So far phy_error() silently stops the PHY state machine. If the network
driver doesn't inform about a  MDIO error then the user may wonder why
his network is down. Let's print the stack trace to facilitate search
for the root cause of the error.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy.c

@@ -730,6 +730,8 @@ void phy_stop_machine(struct phy_device *phydev)
  */
 static void phy_error(struct phy_device *phydev)
 {
+	WARN_ON(1);
+
 	mutex_lock(&phydev->lock);
 	phydev->state = PHY_HALTED;
 	mutex_unlock(&phydev->lock);