ACPI / CPPC: signedness bug in register_pcc_channel()

The "pcc_subspace_idx" is -1 if it hasn't been initialized yet.  We need
it to be signed.

Fixes: 337aadff (ACPI: Introduce CPU performance controls using CPPC)
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/cppc_acpi.c

@@ -302,7 +302,7 @@ int acpi_get_psd_map(struct cpudata **all_cpu_data)
 }
 EXPORT_SYMBOL_GPL(acpi_get_psd_map);
 
-static int register_pcc_channel(unsigned pcc_subspace_idx)
+static int register_pcc_channel(int pcc_subspace_idx)
 {
 	struct acpi_pcct_subspace *cppc_ss;
 	unsigned int len;