Update modbus.py (#8256)

* Update modbus.py

Prevents Modbus binary sensors showing up as "unnamed_device".
Originally proposed here https://community.home-assistant.io/t/modbus-sensor/6751/11 by user Pjeter

* Update modbus.py

homeassistant/components/binary_sensor/modbus.py

@@ -50,6 +50,10 @@ class ModbusCoilSensor(BinarySensorDevice):
         self._coil = int(coil)
         self._value = None
 
+    def name(self):
+        """Return the name of the sensor."""
+        return self._name
+
     @property
     def is_on(self):
         """Return the state of the sensor."""