Apart from the early exploration done to find a console keyboard during

Apart from the early exploration done to find a console keyboard during
the boot process, USB devices must be attached or detached from the usb
task thread in order to avoid races with periodical explorations issued
by uhub(4) interrupts.

Respect this rule when detaching root hubs during a suspend/resume cycle
and avoid some hangs due to the aforementioned race.

Tested by Mattieu Baptiste, thanks!