NVMe: correct comment for offset enum of controller registers in nvme.h

Section 3.1 gives the comment for the offset of controller registers
in the specification 1.2a.

Some are mis-copied in the header file nvme.h. Correct them.

Signed-off-by: Wang Sheng-Hui <shhuiw@foxmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>