nvme: use USEC_PER_SEC instead of magic numbers

Use USEC_PER_SEC instead of magic numbers to make code more readable.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>