Due to the replace of videobuf_read_one to videobuf_read_stream, poll()
method implementation is wrong. This fixes poll() implementation, making
read of /dev/video? to work again.

With this method, an USB driver can use video-buf, without needing to
request memory from the DMA-safe area.
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>