qtcamresolution: fix logic in isValid()
qtcamresolution: fix logic in isValid()

fps is invalid if it is -1 but 0 is allowed (even though it will not make GStreamer happy)

foolab/cameraplus,mlehtima/cameraplus,foolab/cameraplus,alinelena/cameraplus,alinelena/cameraplus,mer-hybris-kis3/cameraplus,mer-hybris-kis3/cameraplus,mlehtima/cameraplus,alinelena/cameraplus,mer-hybris-kis3/cameraplus,foolab/cameraplus,foolab/cameraplus,mlehtima/cameraplus,alinelena/cameraplus,mer-hybris-kis3/cameraplus,mlehtima/cameraplus