media: vivid: remove stream_sliced_vbi_cap field

Vivid tracks the VBI capture mode in vivid_dev->stream_sliced_vbi_cap
field.  We can just look at the buffer type instead, and drop the field.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>