media: ivtv: don't call s_stream(0) if not streaming

Recently a WARN_ON was added in v4l2-subdev.c to warn if s_stream was
called unnecessarily. The ivtv driver hits this once when it is loaded for
the first time. Add a bool to avoid this warning.

[hverkuil: added a comment for the sd_video_is_streaming field]

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>