cpuidle: ACPI: fix 'return' with no value build warning

Add return value to fix return-type build warning introduced by
commit efe9711 ("cpuidle: change enter_s2idle() prototype").

Fixes: efe9711 ("cpuidle: change enter_s2idle() prototype")
Signed-off-by: Neal Liu <neal.liu@mediatek.com>
[ rjw: Subject & changelog edits, make acpi_idle_enter_s2idle()
  return 0 in all cases ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Neal Liu

drivers/acpi/processor_idle.c

@@ -664,11 +664,11 @@ static int acpi_idle_enter_s2idle(struct cpuidle_device *dev,
 		struct acpi_processor *pr = __this_cpu_read(processors);
 
 		if (unlikely(!pr))
-			return;
+			return 0;
 
 		if (pr->flags.bm_check) {
 			acpi_idle_enter_bm(pr, cx, false);
-			return;
+			return 0;
 		} else {
 			ACPI_FLUSH_CPU_CACHE();
 		}