ACPI / scan: fix fixed event handler return value

The fixed event handler should return a value that is either 0 or 1
meanning if the event is handled or not, instead of an acpi_status to
mean if the handler runs well or not.

Suggested-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>