V4L/DVB (5603): V4L: Prevent queueing queued buffers.

videobuf_qbuf queues buffers to q->stream but does not properly check
the state of the buffer before queueing. It was possible to queue
buffers that already were in the queue.
Only buffers that are in states STATE_NEEDS_INIT and STATE_IDLE can be
queued.

Signed-off-by: Sakari Ailus <sakari.ailus@nokia.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>