ACPI LPSS devices use direct-complete style suspend/resume handling by
default. We set the DPM_FLAG_SMART_PREPARE and define a prepare handler
to disable this on Cherry Trail devices.

Clean this up a bit by setting the DPM_FLAG_NO_DIRECT_COMPLETE flag for
Cherry Trail devices, instead of defining a prepare handler.

While at it also improve the comment explaining why this is necessary.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>