nvme: enumerate controller flags

We expect to grow a few of these flags for various purposes
so make them a proper enumeration.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>