fireworks/bebob/oxfw: extend substreams counter for both of PCM/MIDI

Between prepare() and trigger(), there is a time to need amdtp_stream
without PCM pointer. During this time, the other application can stop
streams by calling snd_pcm_close(). As a result, another application do
a bad behaviour in kernel area.

This commit extend current MIDI substreams counter for both of PCM.
With this counter, each stream functionality can stop streams without
reliance on PCM pointer.