die_id is an u16 quantity. On single-die systems the default value of
die_id is 0. No need to check for negative values.

Plus, removing this check makes Coverity happy.
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>