Fix broken QGeoTiledMapData::fitInViewPort()

the zoomfactor was wrong is the minimumZoomLevel is not 0

Task-number: QTBUG-37502
Change-Id: Icc8b561cf1d11c63532c6c8559809e7b00d1ef95
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>