v4l2bufferpool: Don't stop streaming when pool is flushing

v4l2bufferpool: Don't stop streaming when pool is flushing

The purpose of being able to flush the buffer pool is only to
unlock any blocked operation. Doing streamoff/streamon had the
side effect of turning off and on the camera. As we do a flush_start
/ flush_stop sequence when shutting down, that would cause a really
quick sequence of streamoff/streamon/streamoff/close which was
causing some cameras to stop working.

https://bugzilla.gnome.org/show_bug.cgi?id=783945