ACPI: use _HID when supplied by root-level devices

Previously, we assumed the only Device object immediately below the root
was the \_SB Scope (which the ACPI CA treats as a Device), so we forced
the HID of all such objects to ACPI_BUS_HID ("LNXSYBUS").

However, there are DSDTs that supply root-level Device objects with _HIDs.
This patch makes us pay attention to those _HIDs and only add the synthetic
ACPI_BUS_HID for root-level objects that do not supply their own _HID.

For example, this DSDT: https://bugzilla.kernel.org/show_bug.cgi?id=15605
contains:

    Scope (_SB) {
	...
    }
    Device (AMW0) {
	Name (_HID, EisaId ("PNP0C14"))
	...
    }

and we should use "PNP0C14" for the AMW0 device, not "LNXSYBUS".

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Yong Wang <yong.y.wang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/scan.c

@@ -1080,12 +1080,6 @@ static void acpi_device_set_id(struct acpi_device *device)
 		if (ACPI_IS_ROOT_DEVICE(device)) {
 			acpi_add_id(device, ACPI_SYSTEM_HID);
 			break;
-		} else if (ACPI_IS_ROOT_DEVICE(device->parent)) {
-			/* \_SB_, the only root-level namespace device */
-			acpi_add_id(device, ACPI_BUS_HID);
-			strcpy(device->pnp.device_name, ACPI_BUS_DEVICE_NAME);
-			strcpy(device->pnp.device_class, ACPI_BUS_CLASS);
-			break;
 		}
 
 		status = acpi_get_object_info(device->handle, &info);
@@ -1120,6 +1114,12 @@ static void acpi_device_set_id(struct acpi_device *device)
 			acpi_add_id(device, ACPI_DOCK_HID);
 		else if (!acpi_ibm_smbus_match(device))
 			acpi_add_id(device, ACPI_SMBUS_IBM_HID);
+		else if (!acpi_device_hid(device) &&
+			 ACPI_IS_ROOT_DEVICE(device->parent)) {
+			acpi_add_id(device, ACPI_BUS_HID); /* \_SB, LNXSYBUS */
+			strcpy(device->pnp.device_name, ACPI_BUS_DEVICE_NAME);
+			strcpy(device->pnp.device_class, ACPI_BUS_CLASS);
+		}
 
 		break;
 	case ACPI_BUS_TYPE_POWER: