snd-fireworks: fix a bug to set channels when MIDI stream starts AMDTP stream

Even if MIDI stream starts AMDTP stream, the number of PCM channel should be
correctly set into snd_efw structure to read/write data from data blocks.
snd-bebob: modify pcm and stream functions referring to fireworks

Currently bebob driver don't support full-duplex with synchronization because
testing current implementation for the other devices which don't support it.