Set correct output buffer type in vidioc_dqbuf

When userspace dequeues an output buffer from the driver, that buffer should
have type V4L2_BUF_TYPE_VIDEO_OUTPUT. Previously we were providing the type
from our internal buffer which is V4L2_BUF_TYPE_VIDEO_CAPTURE.

(I am trying to send video to the device using v4l2-ctl; it assumes the buffer returned from DQBUF has the correct type, and enqueues it again with QBUF without setting the type, so without this change it doesn't work.)