ACPI / porocessor: Beautify code, pr->id is u32 which is never < 0

pr->id is u32 which never < 0, so remove the redundant pr->id < 0
check from acpi_processor_add().

[rjw: Changelog]
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>