phy: make some bits preserved while setup forced mode

When tested the PHY SGMII Loopback:
1.set the LOOPBACK bit,
2.set the autoneg to AUTONEG_DISABLE, it calls the
genphy_setup_forced which will clear the bit.

The BMCR_LOOPBACK bit should be preserved.

As Florian pointed out that other bits should be preserved too.
So I make the BMCR_ISOLATE and BMCR_PDOWN as well.
Signed-off-by: Weidong Wang <wangweidong1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy_device.c

@@ -1123,8 +1123,9 @@ static int genphy_config_advert(struct phy_device *phydev)
  */
 int genphy_setup_forced(struct phy_device *phydev)
 {
-	int ctl = 0;
+	int ctl = phy_read(phydev, MII_BMCR);
 
+	ctl &= BMCR_LOOPBACK | BMCR_ISOLATE | BMCR_PDOWN;
 	phydev->pause = 0;
 	phydev->asym_pause = 0;