nvme-pci: Free tagset if no IO queues

If a controller becomes degraded after a reset, we will not be able to
perform any IO. We currently teardown previously created request
queues and namespaces, but we had kept the unusable tagset. Free
it after all queues using it have been released.

Tested-by: Edmund Nadolski <edmund.nadolski@intel.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>