decklink: correct framerate 2KDCI 23.98

The impact of this is that prior to this patch setting a decklink device manually to the 2KDCI 23.98 format using the mode flag in the decklinkvideosrc would yield video at around 2 fps, was a minor mistake in the frame rate for this format in the plugin.

This format would have worked normally if the default mode (automatic detection) was used which is why I suspect it wasn't noticed.

Let me know if there clarification or example pipelines are needed, Thanks