ACPI: suppress compiler warnings in processor_throttling.c

This patch fixes following compiler warnings when build via make W=1:

drivers/acpi/processor_throttling.c: In function ‘acpi_processor_throttling_init’:
drivers/acpi/processor_throttling.c:216:40: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>