ALSA: firewire: release reference count of firewire unit in .remove callback of bus driver

In a previous commit, drivers in ALSA firewire stack blocks .remove
callback of bus driver. This enables to release members of private
data in the callback after releasing device of sound card.

This commit simplifies codes to release the members.

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