ALSA: isight: fix leak of reference to firewire unit in error path of .probe callback

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>