xhci: limit the number of link trbs we are willing to process

Needed to avoid we run in circles forever in case the guest builds
an endless loop with link trbs.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Tested-by: P J P <ppandit@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1476096382-7981-1-git-send-email-kraxel@redhat.com