media: mtk-vcodec: venc: fix invalid time per frame in S_PARM

v4l2-compliance expects the driver to adjust the time per frame if it is
invalid (numerator or denominator set to 0). Adjust it to the default
value in these cases.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>