ACPI / scan: constify struct acpi_hardware_id::id

This is preparation for using kstrdup_const to initialize that member.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/acpi/acpi_bus.h

@@ -227,7 +227,7 @@ typedef char acpi_device_class[20];
 
 struct acpi_hardware_id {
 	struct list_head list;
-	char *id;
+	const char *id;
 };
 
 struct acpi_pnp_type {