nvme: rename controller base dev_t char device

Rename controller base dev_t char device in preparation for adding a
namespace char device.

Signed-off-by: Javier González <javier.gonz@samsung.com>
Reviewed-by: Minwoo Im <minwoo.im.dev@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>