nvme-pci: put the admin queue in nvme_dev_remove_admin

Once the controller is shutdown no one can access the admin queue.  Tear
it down in nvme_dev_remove_admin, which matches the flow in the other
drivers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Tested-by Gerd Bayer <gbayer@linxu.ibm.com>