net: stmmac: selftests: Do not fail if PHY is not attached

If a PHY is not attached, we can still run the tests with MAC loopback
mode. Return -EOPNOTSUPP error code in PHY loopback test so that global
status is not a failure.
Signed-off-by: NJose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c

@@ -380,7 +380,7 @@ static int stmmac_test_phy_loopback(struct stmmac_priv *priv)
 	int ret;
 
 	if (!priv->dev->phydev)
-		return -EBUSY;
+		return -EOPNOTSUPP;
 
 	ret = phy_loopback(priv->dev->phydev, true);
 	if (ret)