uvcvideo: Start status polling on device open

From: Laurent Pinchart <laurent.pinchart@skynet.be>

Most UVC camera include an interrupt endpoint to report control value changes,
video streaming errors and camera button events. The USB controller
continuously polls the interrupt endpoint to retrieve such events. This
prevents the device from being auto-suspended, and thus consumes power.

Reporting video streaming errors don't make sense when the V4L2 device is
closed. Control value changes are probably useless as well if nobody listens to
the events, although caching will probably have to be completely disabled then.
No polling is thus be required when /dev/videoX is not opened.

To enable auto-suspend and save power do not poll the interrupt endpoint until
the device is open. We lose the ability to detect button events if no
application is using the camera.

http://bugzilla.kernel.org/show_bug.cgi?id=11948

Priority: normal

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>

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