media: bttv: remove crop info from bttv_buffer

Instead of storing the cropping parameters in each bttv buffer separately,
just use the global bttv crop because it won't change per buffer.

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