acpi_pad: squish warning

drivers/acpi/acpi_pad.c: In function 'power_saving_thread':
drivers/acpi/acpi_pad.c:103: warning: 'preferred_cpu' may be used uninitialized in this function

Cc: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>