Due to the wrong "return" in the loop, a capture substream won't be
released at disconnection properly if the device is capture only and has
no playback substream.  This caused Oops occasionally at the device
reconnection.
Reported-by: Kim Minhyoung <minhyoung.kim@lge.com>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
        for (idx = 0; idx < 2; idx++) {
                subs = &as->substream[idx];
                if (!subs->num_formats)
-                       return;
+                       continue;
                snd_usb_release_substream_urbs(subs, 1);
                subs->interface = -1;
        }