ALSA: firewire-motu: fix destruction of data for isochronous resources

The data for isochronous resources is not destroyed in expected place.
This commit fixes the bug.

Cc: <stable@vger.kernel.org> # v4.12+
Fixes: 9b2bb4f2

 ("ALSA: firewire-motu: add stream management functionality")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>