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().

Signed-off-by: Figo.zhang <figo.zhang@kolorific.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>