usb: ohci: limit the number of link eds

The guest may builds an infinite loop with link eds. This patch
limit the number of linked ed to avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-id: 5899a02e.45ca240a.6c373.93c1@mx.google.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
