media: bttv: replace BUG with WARN_ON

Both BUG and BUG_ON are replaced with WARN_ON wherever they would still be
present after the vb2 conversion. WARN_ON is sufficient in these cases.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>