ACPICA: Constify pathname argument for acpi_get_handle()

acpi_get_handle() uses the pathname argument to find a handle related to
that pathname but it does not need to modify it. Make it const, in order
to be able to pass const pathname to it.

Link: https://github.com/acpica/acpica/pull/773
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>