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>