platform/x86: wmi: more detailed error reporting in find_guid()

Make `find_guid()` return an acpi_status, and make it handle NULL
pointer GUID strings; and adapt users accordingly.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Link: https://lore.kernel.org/r/20210904175450.156801-31-pobrn@protonmail.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>