media: uvcvideo: Remove stream->is_streaming field

The is_streaming field is used by modular PM to know if the device is
currently streaming or not.

With the transition to vb2 and fop helpers, we can use vb2 functions for
the same functionality. The great benefit is that vb2 already tracks the
streaming state for us.

Reviewed-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://lore.kernel.org/r/20250616-uvc-fop-v4-4-250286570ee7@chromium.org


Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>