[media] cx88: v4l2-compliance fixes

Fix three v4l2-compliance failures:

- the colorspace wasn't set in vidioc_try_fmt_vid_cap().
- the field wasn't set in v4l2_buffer when vb2_buffer_done() was called.
- the sequence wasn't set in v4l2_buffer when vb2_buffer_done() was called.
  This fix also removes the unused buf->count field and starts the count
  at 0 instead of 1.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>