media: mtk-vcodec: venc: set default time per frame

The time per frame was left initialized to 0/0, which make the driver
fail v4l2-compliance, and also leaves it potentially exposed to doing a
division by zero.

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>