ACPI / sysfs: correctly check failing memory allocation

Since kobject_create_and_add() can fail under memory pressure,
its return value needs to be checked against NULL before passing
it to sysfs_create_file().

Signed-off-by: Insu Yun <wuninsu@gmail.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>