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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/phy.c

···
  */
 static void phy_error(struct phy_device *phydev)
 {
+	WARN_ON(1);
+
 	mutex_lock(&phydev->lock);
 	phydev->state = PHY_HALTED;
 	mutex_unlock(&phydev->lock);