net: phy: execute genphy_loopback() per default on all PHYs

The generic loopback is really generic and is defined by the 802.3
standard, we should just mandate that drivers implement a custom
loopback if the generic one cannot work.
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy_device.c

@@ -1777,6 +1777,9 @@ int phy_loopback(struct phy_device *phydev, bool enable)
 	struct phy_driver *phydrv = to_phy_driver(phydev->mdio.dev.driver);
 	int ret = 0;
 
+	if (!phydrv)
+		return -ENODEV;
+
 	mutex_lock(&phydev->lock);
 
 	if (enable && phydev->loopback_enabled) {
@@ -1789,10 +1792,10 @@ int phy_loopback(struct phy_device *phydev, bool enable)
 		goto out;
 	}
 
-	if (phydev->drv && phydrv->set_loopback)
+	if (phydrv->set_loopback)
 		ret = phydrv->set_loopback(phydev, enable);
 	else
-		ret = -EOPNOTSUPP;
+		ret = genphy_loopback(phydev, enable);
 
 	if (ret)
 		goto out;