V4L/DVB (6278): Buf: fix typo that caused data loss when readng streams from device

If videobuf_read_stream reads two or more buffers it was overwriting the first one

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
http://thread.gmane.org/gmane.comp.video.video4linux/34978/focus=34981


Reviewed-by: Ricardo Cerqueira <v4l@cerqueira.org>