v4l2: Fix H.264 level 3 string representation
v4l2: Fix H.264 level 3 string representation

The string_to_level function handles "3" so the level_to_string function should
do the same, to prevent caps negotiation issues.

GStreamer/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,pexip/gst-plugins-good