media: bttv: use video_drvdata to get bttv

Use video_drvdata(file) instead of fh->btv to get the bttv pointer
wherever the bttv pointer will still be needed after the vb2 conversion.

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