media: au0828: stop video streaming only when last user stops

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>