ACPI / property: avoid leaking format string into kobject name

The dn->name is expected to be used as a literal, so add the missing
"%s".

Fixes: 263b4c1a

 (ACPI / property: Expose data-only subnodes via sysfs)
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>