nvme-pci: make nvme_dev_pm_ops static

Fix sparse warning:

drivers/nvme/host/pci.c:2926:25: warning:
 symbol 'nvme_dev_pm_ops' was not declared. Should it be static?
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Reviewed-by: NMinwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>

drivers/nvme/host/pci.c

@@ -2923,7 +2923,7 @@ static int nvme_simple_resume(struct device *dev)
 	return 0;
 }
 
-const struct dev_pm_ops nvme_dev_pm_ops = {
+static const struct dev_pm_ops nvme_dev_pm_ops = {
 	.suspend	= nvme_suspend,
 	.resume		= nvme_resume,
 	.freeze		= nvme_simple_suspend,