net: dsa: Do not override PHY interface if already configured

In case we need to divert reads/writes using the slave MII bus, we may have
already fetched a valid PHY interface property from Device Tree, and that
mode is used by the PHY driver to make configuration decisions.

If we could not fetch the "phy-mode" property, we will assign p->phy_interface
to PHY_INTERFACE_MODE_NA, such that we can actually check for that condition as
to whether or not we should override the interface value.

Fixes: 19334920ea ("net: dsa: Set valid phy interface type")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -756,7 +756,8 @@ static int dsa_slave_phy_connect(struct dsa_slave_priv *p,
 		return -ENODEV;
 
 	/* Use already configured phy mode */
-	p->phy_interface = p->phy->interface;
+	if (p->phy_interface == PHY_INTERFACE_MODE_NA)
+		p->phy_interface = p->phy->interface;
 	phy_connect_direct(slave_dev, p->phy, dsa_slave_adjust_link,
 			   p->phy_interface);