tools/power/x86/intel-speed-select: Enforce isst_id value

Enforce the pkg/die value in struct isst_id are either -1 or a valid
value.

This helps avoid inconsistent or redundant checks.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>