media: bttv: remove tvnorm field from bttv_buffer

Instead of storing the tvnorm in each bttv buffer separately, just use the
global bttv tvnorm because the tvnorm does not change per buffer.

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