[SCSI] fix sdev_rw_attr macro for scsi device sysfs entries

This patch fixes sdev_rw_attr() macro for scsi device sysfs entries.
It seems there is no such function snscanf in the current linux kernel,
so it fails to compile scsi driver when someone try to add a new rw entry.
This has been unfixed for a long time probably because current scsi device
has no rw entries.

# grep snscanf . -rn
./drivers/scsi/scsi_sysfs.c:489:        snscanf (buf, 20, format_string, &sdev->field);                 \

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@jp.fujitsu.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>