ALSA: firewire-lib: leave unit reference counting completely

With previous commit, this module managed to leave the counting to each
drivers, but the isochronous resources functionality still increment/decrement
the count.

This commit purge such codes to leave the responsibility to each drivers.

Fix: c6f224dc

 ('ALSA: firewire-lib: remove reference counting')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>