Make xhci compile.
Calculate the Route String when attaching a new device.  This is still
not enough to attach Super Speed devices below USB 3 hubs, but we're
getting there.

While here reset `acten` when re-enqueuing an interrupt transfers.
Do not enable interrupts before attaching usb(4), fix a panic when an
Express Card is plugged with USB devices on it.

While here do not print an unitialized error value if xhci_init() fails,
from Patrick Wildt.