ptp: replace snprintf with sysfs_emit

coccinelle report:
./drivers/ptp/ptp_sysfs.c:17:8-16:
WARNING: use scnprintf or sprintf
./drivers/ptp/ptp_sysfs.c:390:8-16:
WARNING: use scnprintf or sprintf

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: David Yang <davidcomponentone@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>