XHCI_DOORBELL
XDWRITE4(sc, XHCI_DOORBELL(0), 0);
XDWRITE4(sc, XHCI_DOORBELL(0), 0);
XDWRITE4(sc, XHCI_DOORBELL(xp->slot), xp->dci);
XDWRITE4(sc, XHCI_DOORBELL(xp->slot), xp->dci);
XDWRITE4(sc, XHCI_DOORBELL(xp->slot), xp->dci);