nvme-pci: unquiesce admin queue on shutdown

Just like IO queues, the admin queue also will not be restarted after a
controller shutdown. Unquiesce this queue so that we do not block
request dispatch on a permanently disabled controller.

Reported-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>