If the application was streaming from both videoX and vbiX, and streaming
from videoX was stopped, then the vbi streaming also stopped.

The cause being that stop_streaming for video stopped the subdevs as well,
instead of only doing that if dev->streaming_users reached 0.

au0828_stop_vbi_streaming was also wrong since it didn't stop the subdevs
at all when dev->streaming_users reached 0.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>