ahal: return proper read size when SVA is internally stopped

For dualVA, when one model is unloaded during lab reading for
another model, graph may be restarted and no further lab data
can be provided. In this case we should return read data as
zeros and do not notify any error to framework side.

Return requested size if SVA lab read fails.

Change-Id: I48fe70708107fc6a975dd27b1dca8b273bafdfa7