ACPI / processor: use acpi_evaluate_ost() to replace open-coded version

Use public function acpi_evaluate_ost() to replace open-coded
version of evaluating ACPI _OST method.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>