media: uvcvideo: Turn on the camera if V4L2_EVENT_SUB_FL_SEND_INITIAL

media: uvcvideo: Turn on the camera if V4L2_EVENT_SUB_FL_SEND_INITIAL

If we subscribe to an event with V4L2_EVENT_SUB_FL_SEND_INITIAL, the
driver needs to report back some values that require the camera to be
powered on. But VIDIOC_SUBSCRIBE_EVENT is not part of the ioctls that
turn on the camera.

We could unconditionally turn on the camera during
VIDIOC_SUBSCRIBE_EVENT, but it is more efficient to turn it on only
during V4L2_EVENT_SUB_FL_SEND_INITIAL, which we believe is not a common
usecase.

To avoid a list_del if uvc_pm_get() fails, we move list_add_tail to the
end of the function.

Reviewed-by: Hans de Goede <hansg@kernel.org>
Fixes: d1b618e79548 ("media: uvcvideo: Do not turn on the camera for some ioctls")
Cc: stable@vger.kernel.org
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://lore.kernel.org/r/20250701-uvc-grannular-invert-v4-5-8003b9b89f68@chromium.org
Signed-off-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>