v4l2: call VIDIOC_REQBUFS with count = 0 in pool_finalize

v4l2: call VIDIOC_REQBUFS with count = 0 in pool_finalize

Without this the following sequence fails:

- set_caps()
  - object_stop() (does nothing)
  - set_format() -> VIDIOC_S_FMT
- set_config() -> VIDIOC_REQBUFS with count = N
- set_caps()
  - object_stop()
    - pool_finalize()
  - set_format() -> VIDIOC_S_FMT => EBUSY

Usually the pool is started after set_config(), in which case object_stop()
will result in a pool_stop and therefore VIDIOC_REQBUFS with count = 0 but
that is not guaranteed.
Also calling VIDIOC_REQBUFS with count = 0 in pool_finalize() if necessary
fixes this problem.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=701543