There is an issue where the host is unable to tell the gadget what frame
rate it wants if the dwFrameIntervals in the interface descriptors are
not in ascending order. This means that when instantiating a uvc gadget
via configfs the user must make sure the dwFrameIntervals are in
ascending order.

Instead of silently failing the breaking of this rule, we sort the
dwFrameIntervals upon writing to configfs.
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>