nvme: use sysfs_emit instead of sprintf

sysfs_emit is the recommended API to use for formatting strings to be
returned to user space. It is equivalent to scnprintf and aware of the
PAGE_SIZE buffer size.

Suggested-by: Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>