net: phy: core: use genphy version of callbacks read_status and config_aneg per default

read_status and config_aneg are the only mandatory callbacks and most
of the time the generic implementation is used by drivers.
So make the core fall back to the generic version if a driver doesn't
implement the respective callback.

Also currently the core doesn't seem to verify that drivers implement
the mandatory calls. If a driver doesn't do so we'd just get a NPE.
With this patch this potential issue doesn't exit any longer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/phy/phy.c b/drivers/net/phy/phy.c
index 2b1e67b..a0e7605 100644
--- a/drivers/net/phy/phy.c
+++ b/drivers/net/phy/phy.c
@@ -493,7 +493,10 @@ static int phy_start_aneg_priv(struct phy_device *phydev, bool sync)
 	/* Invalidate LP advertising flags */
 	phydev->lp_advertising = 0;
 
-	err = phydev->drv->config_aneg(phydev);
+	if (phydev->drv->config_aneg)
+		err = phydev->drv->config_aneg(phydev);
+	else
+		err = genphy_config_aneg(phydev);
 	if (err < 0)
 		goto out_unlock;