media: bttv: remove format field from bttv_buffer

Instead of storing the format (video or vbi) in each bttv buffer
separately, just use the global bttv format because the format does not
change per buffer.

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