videoparseutils: Fix for wrong CEA708 minimum size check

The minimum possible size of valid CEA708 data is 3 bytes, not 7 bytes

Part-of: <gstreamer/gst-plugins-bad!2507>
33 jobs for !2507 with backport-mr-2505-into-1.18-parse-cea708-size in 30 minutes and 1 second (queued for 4 seconds)
merge request