kernel/usb4bsd: Port run(4), add firmware too.

Some minor issues might remain, but it works good here with various
compatible adapters.

While here, sync with FreeBSD's latest.

Taken-from: FreeBSD
kernel/usb4bsd: Remove unnecessary NULL checks in rum(4) and run(4).
kernel/usb4bsd: Warning fix and minor adjustments from FreeBSD for xhci.