V4L/DVB: Remove videobuf_sg_alloc abuse

The cx88 and cx25821 drivers abuse videobuf_buffer to handle audio data.
Remove the abuse by creating private audio buffer structures with a
videobuf_dmabuf field.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>