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

Add return value to fix return-type build warning introduced by
commit efe97112 ("cpuidle: change enter_s2idle() prototype").

Fixes: efe97112 ("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>