st: convert DRIVER_ATTR macros to DRIVER_ATTR_RO

Convert DRIVER_ATTR macros to DRIVER_ATTR_RO requested by
Greg KH. Also switched to using scnprintf instead of snprintf
per Documentation/filesystems/sysfs.txt.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shane Seymour <shane.seymour@hp.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Odin.com>