ALSA: firewire-motu: unify the count of subscriber for packet streaming

Two counters are used to maintain isochronous packet streaming for both
directions. However, like the other drivers, they can be replaced with
one counter. This commit unifies them.

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