[media] au0828: fix broken streaming

Commit c5036d61 ("media: au0828: drop
vbi_buffer_filled() and re-use buffer_filled()") broke video and vbi streaming.

The vb2_buffer struct was copied instead of taking a pointer to it, but
vb2_buffer_done() needs the real object, not a copy, since it is hooking
the buffer into a different list.

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