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>

drivers/acpi/processor_perflib.c

@@ -70,7 +70,7 @@ static DEFINE_MUTEX(performance_mutex);
  *  0 -> cpufreq low level drivers initialized -> consider _PPC values
  *  1 -> ignore _PPC totally -> forced by user through boot param
  */
-static unsigned int ignore_ppc = -1;
+static int ignore_ppc = -1;
 module_param(ignore_ppc, uint, 0644);
 MODULE_PARM_DESC(ignore_ppc, "If the frequency of your machine gets wrongly" \
 		 "limited by BIOS, this should help");