ALSA: fireworks: unify substream counter

This commit is a part of preparation to perform allocation/release
of isochronous resources in pcm.hw_params/hw_free callbacks.

It's inconvenient to handle two isochronous context separately
each other. This commit unifies the counters to handle the two
at the same time.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>