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>

drivers/nvme/host/core.c

@@ -2947,7 +2947,7 @@ int nvme_init_identify(struct nvme_ctrl *ctrl)
 
 	if (id->rtd3e) {
 		/* us -> s */
-		u32 transition_time = le32_to_cpu(id->rtd3e) / 1000000;
+		u32 transition_time = le32_to_cpu(id->rtd3e) / USEC_PER_SEC;
 
 		ctrl->shutdown_timeout = clamp_t(unsigned int, transition_time,
 						 shutdown_timeout, 60);