xhci: only free completed transfers

Most callsites check already, one was missed.

Cc: 1653384@bugs.launchpad.net
Fixes: 94b037f2a451b3dc855f9f2c346e5049a361bd55
Reported-by: Fabian Lesniak <fabian@lesniak-it.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1485790607-31399-2-git-send-email-kraxel@redhat.com
