platform/x86: thinkpad_acpi: Accept flat mode for type 4 multi mode status

On the X1 Yoga 2nd Generation and most likely other notebooks the FLAT
mode is reported. Decode it correctly rather than warning about an
unexpected multi mode status to be reported.

Signed-off-by: Benjamin Berg <bberg@redhat.com>
Cc: Peter FP1 Zhang <zhangfp1@lenovo.com
Cc: Lyude <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>