ACPI: acpi_pad: Fix memory leak in power saving threads

Fix once per second (round_robin_time) memory leak of about 1 KB in
each acpi_pad kernel idling thread that is activated.

Found by testing with kmemleak.

Signed-off-by: Lenny Szubowicz <lszubowi@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>