nvme: Do a clean NVMe shutdown

The brute-force controller disable method can end up racing controller
initialization and causing a crash when we shut down Apple ANS2 NVMe
controllers. Do a proper controlled shutdown, which does block until
things are quiesced properly. This is nicer in general for all
controllers.

Signed-off-by: Hector Martin <marcan@marcan.st>
Tested-by: Mark Kettenis <kettenis@openbsd.org> (firefly-rk3399)