[media] vb2: Return 0 when streamon and streamoff are already on/off

According to the doc:
If VIDIOC_STREAMON is called when streaming is already in progress,
or if VIDIOC_STREAMOFF is called when streaming is already stopped,
then the ioctl does nothing and 0 is returned.
The current implementation was returning -EINVAL instead.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>