ACPI: Fix thermal shutdowns

Do not use unsigned int if there is test for negative number...

See drivers/acpi/processor_perflib.c
  static unsigned int ignore_ppc = -1;
...
  if (event == CPUFREQ_START && ignore_ppc <= 0) {
       ignore_ppc = 0;
...

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>