nvme: use ctrl state accessor

The ctrl->state value is updated in another thread using WRITE_ONCE, so
ensure all the readers use the appropriate accessor.

Reviewed-by: Sagi Grimberg <sagi@grmberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>