ACPI / scan: constify first argument of struct acpi_scan_handler::match

One wouldn't expect a "match" function modify the string it searches
for, and indeed the only instance of the struct
acpi_scan_handler::match callback, acpi_pnp_match, can easily be
changed. While there, update its helper matching_id().

This is also preparation for constifying struct acpi_hardware_id::id.

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