Re: kern/53598: assertion failure in usb detach

[Martin Husemann <martin%netbsd.org@localhost>]

Problem is that this (VirtualBox) xhci only partly attached, USB revision
is 0 and we never make it to usb_doattach(), but set sc_dying instead.

When we get to usb_detach, the softint never had been established
nor the cv initialized.

Maybe we should define several values for sc_dying, representing how
far during initalization we got? Something like: sc_dying = 3 if we
did not initialize anything, sc_dying = 2 if softint is valid but
cv is not, sc_dying = 1 if we have been fully functional but are now
winding down.

Martin