The change_state function is called synchronously from the
image_captured callback. This means that deactivation of the device
happens during the img_cb function, causing the USB transfer to be
re-registered even though the device is already deactivating.
There are various ways to fix this, but it makes sense to directly bind
the cancellation to the deactivation. So create a cancellable that we
cancel at deactivation time, and make sure we always deactivate by going
through cancellation.
closes: #306
