data getter in Suspense component has side effects

[elwayman02]

https://github.com/sunishsheth2009/ember-async-component/blob/master/addon/components/suspense/index.js#L81-L85

[elwayman02]

@sunishsheth2009 do you have any thoughts on how we can remove those two sets from this getter? Can we somehow turn task and promise into their own getters?