nvme: fix status magic numbers

Replaced some magic numbers about SC and SCT with enum and macro.

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