fix conditionals in DayTimeDuration::stringValue()

Task-number: QTBUG-35479
Change-Id: If385bd85f5c20871a0248c828626deea2d72e0a2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>