v4l2object: use bt601 transfer function for SMPTE170M
Commit 1d0ccf8baaf4 ("video-color: Add bt601 transfer function") [0] adds a separate transfer function for bt.601 instead of reusing the id of the 709 transfer function. Therefore, the V4L2_COLORSPACE_SMPTE170M does not match the bt601 colorimetry string anymore.
Set the transfer function to GST_VIDEO_TRANSFER_BT601 if V4L2_COLORSPACE_SMPTE170M is found to identify it as bt601.