ahal: return proper read size for sva lab reading

When sva client is reading lab data, it is possible
that actual read size may be smaller than expected
size as SVA buffering is non-blocking read. But in
hal we see expected size is always returned as read
size, causing improper data included in lab pcm.

Return correct size for sva lab reading.

Change-Id: I1cf605fbeb6c9f6356aa1c6fbbbfb94059832bcd