usb-ehci: remove hack

To answer the question in the comment removed by this patch:  I think
this was needed because several places in the ehci emulation did not
check the T bit of link entries correctly and thus might have followed
invalid references.  See commit 2a5ff735dc1074171a0cbb1dc228d6d6e907f571

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
