usbdi(9): Suspend control pipe on detach.

The device is gone so control transfers won't complete anyway.  This
obviates the need to wait for usbd_do_request to time out.

Seems like maybe we should make _all_ xfers fail with USBD_CANCELLED
when the device is detached, but there's no list of pipes we can just
walk down to suspend them, so we'd have to find another way to do so.
For now, we'll just keep having drivers suspend/abort pipes other
than the control pipe.

(riastradh)