Small cleanups of the soft clipping code.

- Made OP_SOFT_CLIP defined only when using floating point (libopus
   handles clipping when using the fixed-point API).
- Struct rearrangement for theoretically better packing on some
   platforms.
- Reset the state tracking channel count when re-initializing a
   decoder (and not the dither mute, because that gets reset for
   free).