directsoundsrc: Correctly calculate segsize and segtotal

directsoundsrc: Correctly calculate segsize and segtotal

segsize should be based on latency-time, and must be a multiple of the
frame size. segtotal should be based on buffer-time and segsize.

This prevents errors caused by outputting buffers that are not a
multiple of the frame size, and actually makes the buffer-time and
latency-time properties do what they're supposed to do.