wmi: Free the allocated acpi objects through wmi_get_event_data

These function allocate an acpi object by calling wmi_get_event_data, which
then calls acpi_evaluate_object, and it is not freed afterwards.

And kernel doc is fixed for parameters of wmi_get_event_data.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>