fix min data length for custom functions (#594)

Author: BryanCrotazGivEnergy

ports/tcpport.js

@@ -9,7 +9,7 @@ const crc16 = require("../utils/crc16");
 /* TODO: const should be set once, maybe */
 const MODBUS_PORT = 502; // modbus port
 const MAX_TRANSACTIONS = 256; // maximum transaction to wait for
-const MIN_DATA_LENGTH = 6;
+const MIN_DATA_LENGTH = 4; // custom function can have length 4
 const MIN_MBAP_LENGTH = 6;
 const CRC_LENGTH = 2;