v4l2object: handle GST_VIDEO_TRANSFER_BT601

v4l2object: handle GST_VIDEO_TRANSFER_BT601

V4L2 makes no difference between the BT.601 and BT.709 transfer
functions [1], but GStreamer does since 1.18 [2].

Adapt gst_v4l2_object_get_colorspace() and
gst_v4l2_object_set_format_full().

[1] https://linuxtv.org/downloads/v4l-dvb-apis-new/userspace-api/v4l/colorspaces-details.html#colorspace-smpte-170m-v4l2-colorspace-smpte170m
[2] https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/724

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/856>