fireworks/bebob/oxfw: add reference counter for MIDI substream

I realized 'amdtp_midi_running()' is meaningless.

Originally, 'amdtp_stream' structure keep pointers for MIDI substream when
substreams are open in driver side. But now this idea is not used. Then the
function is meaningless.

Here, I add reference counter for MIDI substream for incurnations for each
device. This reference counter should be incremented when opening a MIDI
substream, should be decreased when closing a MIDI substream. Each streaming
functionality can stop streams correctly referring to the counter.

I've considered about counter for both of PCM/MIDI. But in PCM functionality,
snd_pcm_prepare() can be called several times. Then counter management is
difficult.
firewire-lib: remove 'amdtp_stream_midi_running()'

This is obsoleted.