usb-uhci: fix race against host controller

usb-uhci: fix race against host controller

While processing a frame, the host controller will write to the queue
head's element link field.  The following sequence could then happen
when two consecutive sends occur to the same pipe.

    controller                      SeaBIOS
    ---------------------------------------------------------------------
                                    td->link = UHCI_PTR_TERM;
                                    td->ctrl |= TD_CTRL_ACTIVE;
    read TD from memory
                                    wait_td(td);
    td->ctrl &= ~TD_CTRL_ACTIVE;
    write back td->ctrl
                                    exit usb_send_bulk
                                    restart usb_send_bulk
                                    pipe->qh.element = &tds;
    pipe->qh.element = td->link;    ... go on and set up the first td ...
    write back pipe->qh.element
                                    td->ctrl |= TD_CTRL_ACTIVE;

Once the host controller has written UHCI_PTR_TERM to the element link,
subsequent tds would never be processed.  This is surprisingly frequent
when the two consecutive sends are in the OUT direction (and just as
surprisingly, it seems like it never happens in the IN direction).

To fix this, at the end of the processing do not wait for each single
TD to become inactive, but for the host controller to invalidate the
element link (which implies it's done with all TDs).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>