v4l2: Don't start read buffers when stream is off.

When polling streams, if stream is already off, we
don't re-construct the queue and start reading. Return
POLLERR instead. This eliminates the race condition
where after streamoff, application tries to pull a
frame buffer out.

Change-Id: I86e13500f20add58ee6eca8b001bf557e66fd996
Signed-off-by: Shuzhen Wang <shuzhenw@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>