added payload length for Max child timeout tlv (ARMmbed#1856)

Author: deepakvenugopal

source/6LoWPAN/Thread/thread_diagnostic.c

@@ -229,6 +229,10 @@ static int thread_diagnostic_configuration_calc(protocol_interface_info_entry_t
             payload_len += 2+1;
             break;
 
+        case DIAGCOP_TLV_MAX_CHILD_TIMEOUT:
+            payload_len += 2 + 4;
+            break;
+
         default:
             // todo: Other TLV's not supported atm
             break;