ALSA: ua101: fix crash when unplugging

If the device is unplugged while running, it is possible for a PCM
device to be closed after the disconnect callback has returned.  This
means that kill_stream_urb() and disable_iso_interface() would try to
access already-invalid or freed USB data structures.

The function free_usb_related_resources() was intended to prevent this,
but forgot to clear the affected variables.

Reported-and-tested-by: Olivier Courtay <olivier@courtay.org>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: 2.6.33+ <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>