media: bttv: start_streaming should return a proper error code

The start_streaming callback returned 0 or 1 instead of a
proper error code. Fix that.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: b7ec3212 ("media: bttv: convert to vb2")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>