Fixed NDPU comparison functions that were missing segment-ack PDU. (#991)

ports/linux/dlmstp_port.c

@@ -471,6 +471,7 @@ static bool dlmstp_compare_data_expecting_reply(
             break;
         case PDU_TYPE_REJECT:
         case PDU_TYPE_ABORT:
+        case PDU_TYPE_SEGMENT_ACK:
             reply.invoke_id = reply_pdu[offset + 1];
             break;
         default:
@@ -478,7 +479,8 @@ static bool dlmstp_compare_data_expecting_reply(
     }
     /* these don't have service choice included */
     if ((reply.pdu_type == PDU_TYPE_REJECT) ||
-        (reply.pdu_type == PDU_TYPE_ABORT)) {
+        (reply.pdu_type == PDU_TYPE_ABORT) ||
+        (reply.pdu_type == PDU_TYPE_SEGMENT_ACK)) {
         if (request.invoke_id != reply.invoke_id) {
             debug_printf("DLMSTP: DER Compare failed: "
                          "Invoke ID mismatch.\n");