the device is suspended or shutting down. This will need to be rethought
slightly if we implement suspend/resume support within vr(4).
This appears to fix the vr_shutdown() panic on SMP machines.
My theory here is there's a race somewhere during vr_detach() with
vr_intr() in the SMP case which was sometimes being triggered,
although quite why this was happening is unclear (vr_stop() also
explicitly disables interrupts by writing to the IMR register).
MFC-to-RELENG_5* candidate.
PR: kern/62889
Tested by: seb at struchtrup dot com
MFC after: 10 days
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