sandbox: fix sound driver

In the callback function we have to use memcpy(). Otherwise we add
the new samples on top of what is stored in the stream buffer.

If we don't have enough data, zero out the rest of the stream buffer.

Our sampling frequency is 48000. Let the batch size for the callback
function be 960. If we play a multiple of 20 ms, this will always be
a full batch.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>