v4l2: Avoid code duplication

The function gst_v4l2_object_add_interlace_mode() has repeating code so
it's best use a loop instead. That will make it easy and simple to add
additional interlace modes in a following patch.
10 jobs for v4l2-cleanup in 39 minutes and 51 seconds (queued for 4 minutes and 32 seconds)