eeepc-wmi: add an helper using simple return codes

eeepc_wmi_get_devstate returns an acpi_status, so each
call need extra logic to handle the return code. This
patch add a simple getter, returning a boolean (or a
negative error code).

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Matthew Garrett <mjg@redhat.com>