[modbus] shift orders for negative values

pyheatpump/modbus.py

@@ -134,10 +134,14 @@ def write_holding_register(var_value):
     )
 
     try:
+        casted_value = int(var_value.value) + (1 << 16)
+            if (var_value.value < 0)
+            else int(var_value.value)
+
         req_adu = rtu.write_single_register(
             slave_id=1,
             address=var_value.address,
-            value=var_value.value)
+            value=casted_value)
         response = rtu.send_message(req_adu, serial_conn)
         logger.debug('write_holding_register, addres: %s, response: %s',
                 var_value.address, response)