In some error paths, reference count of firewire unit is not decreased.
This commit fixes the bug.

Fixes: 5b14ec25('ALSA: firewire: release reference count of firewire unit in .remove callback of bus driver')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>