kern/54963: UHCI panic on shutdown

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         54963
>Category:       kern
>Synopsis:       UHCI panic on shutdown
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Feb 14 14:20:01 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

My amd64 bare metal testbed has recently started panicing in the
uhci(4) driver on shutdown:

  [ 7185.0687237] panic: kernel diagnostic assertion "sc->sc_intr_xfer == NULL" failed: file "/tmp/bracket/build/2020.02.13.00.42.59-amd64-baremetal/src/sys/dev/usb/uhci.c", line 3928

Full console log including boot messages and panic message:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.02.13.00.42.59/test.log

Source date 2020.02.11.07.03.16 shuts down cleanly, 2020.02.13.00.42.59 panics.

>How-To-Repeat:

>Fix: