ACPI: fix a regression of ACPI device driver autoloading

commit 3620f2f2 sets the cid of
ACPI video/dock/bay device and leaves the hid empty.
As a result, "modalias" should export the cid for
devices which don't have a hid.

ACPI Video driver is not autoloaded with
commit 3620f2f2 applied.
"cat /sys/.../device:03(acpi video bus)/modalias" shows nothing.

ACPI Video driver is autoloaded after revert that commit.
"cat /sys/.../LNXVIDEO:0x/modalias" shows "acpi:LNXVIDEO:"

ACPI Video driver is autoloaded with commit
3620f2f2 and this patch applied.
"cat /sys/.../device:03(acpi video bus)/modalias"
shows "acpi:LNXVIDEO:"
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/scan.c

@@ -39,20 +39,26 @@ static int create_modalias(struct acpi_device *acpi_dev, char *modalias,
 			   int size)
 {
 	int len;
+	int count;
 
-	if (!acpi_dev->flags.hardware_id)
+	if (!acpi_dev->flags.hardware_id && !acpi_dev->flags.compatible_ids)
 		return -ENODEV;
 
-	len = snprintf(modalias, size, "acpi:%s:",
+	len = snprintf(modalias, size, "acpi:");
+	size -= len;
+
+	if (acpi_dev->flags.hardware_id) {
+		count = snprintf(&modalias[len], size, "%s:",
 				 acpi_dev->pnp.hardware_id);
-	if (len < 0 || len >= size)
+		if (count < 0 || count >= size)
 			return -EINVAL;
-	size -= len;
+		len += count;
+		size -= count;
+	}
 
 	if (acpi_dev->flags.compatible_ids) {
 		struct acpi_compatible_id_list *cid_list;
 		int i;
-		int count;
 
 		cid_list = acpi_dev->pnp.cid_list;
 		for (i = 0; i < cid_list->count; i++) {