ACPI / scan: use kstrdup_const() in acpi_add_id()

Empirically, acpi_add_id is mostly called with string literals, so
using kstrdup_const for initializing struct acpi_hardware_id::id saves
a little run-time memory and a string copy.

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