V4L/DVB (11990): saa7134-video.c: fix the block bug
when re-open or re-start (video_streamon), the q->curr would not be NULL in saa7134_buffer_queue(),
and all the qbuf will add to q->queue list,no one to do activate to start DMA,and then no interrupt
would happened,so it will be block.
In VIDEOBUF_NEEDS_INIT state, initialize the curr pointer to be NULL in the buffer_prepare().