V4L/DVB (6274): V4L: vivi.c replace logic in vivi_poll with videobuf_poll_stream

Since vivi is using videobuf_read_stream() it can use videobuf_poll_stream()
now.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>