Expose quantization setting

Can be changed with
- command line "v4l2-ctl -d /dev/video1 -c h264_minimum_qp_value=0",
- ioctl VIDIOC_S_CTRL or VIDIOC_S_EXT_CTRLS with id V4L2_CID_MPEG_VIDEO_H264_MIN_QP.
Expose GOP size setting for EXT_CTRLS calls

Also drop the check of number of readers at its value checking