media: vb2/au0828: move the v4l_vb2q_enable_media_source to the au0828 driver

With the new prepare_streaming op it is possible to move the ugly
v4l_vb2q_enable_media_source() call in vb2_core_streamon to the
driver. It was called incorrectly in vb2 as well: it was only
called if sufficient buffers were queued at STREAMON time, but not
if more buffers were queued later. This was not an issue with the
au0828 driver since it never set min_buffers_needed.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>