v4l2: Avoid code duplication
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.

GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good