ACPI / processor: Finish making acpi_processor_ppc_has_changed() void

Commit bca5f557 "ACPI / processor: Make acpi_processor_ppc_has_changed()
void" changed one of the declarations of acpi_processor_ppc_has_changed()
to return void, but the !CPU_FREQ version still returns int. Let's return
void to be consistent.

Fixes: bca5f557 "ACPI / processor: Make acpi_processor_ppc_has_changed() void"
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>