usb: core: phy: clean up return value check about devm_of_phy_get_by_index()

Use IS_ERR() instead of IS_ERR_OR_NULL() because devm_of_phy_get_by_index()
never return NULL value;
But still need ignore the error of -ENODEV, for more information, please
refer to:
[0] https://lkml.org/lkml/2018/4/19/88
[1] https://patchwork.kernel.org/patch/10160181/

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/phy.c

@@ -23,10 +23,11 @@ static int usb_phy_roothub_add_phy(struct device *dev, int index,
 				   struct list_head *list)
 {
 	struct usb_phy_roothub *roothub_entry;
-	struct phy *phy = devm_of_phy_get_by_index(dev, dev->of_node, index);
+	struct phy *phy;
 
-	if (IS_ERR_OR_NULL(phy)) {
-		if (!phy || PTR_ERR(phy) == -ENODEV)
+	phy = devm_of_phy_get_by_index(dev, dev->of_node, index);
+	if (IS_ERR(phy)) {
+		if (PTR_ERR(phy) == -ENODEV)
 			return 0;
 		else
 			return PTR_ERR(phy);