Fix supported_features in mqtt cover (#28120)

* Correctly compute the supported_features in cover.mqtt

* Update homeassistant/components/mqtt/cover.py

Co-Authored-By: Paulus Schoutsen <paulus@home-assistant.io>

* Correctly compute the supported_features in cover.mqtt

* Format

tests/components/mqtt/test_cover.py

@@ -546,6 +546,27 @@ async def test_no_command_topic(hass, mqtt_mock):
     assert hass.states.get("cover.test").attributes["supported_features"] == 240
 
 
+async def test_no_payload_stop(hass, mqtt_mock):
+    """Test with no stop payload."""
+    assert await async_setup_component(
+        hass,
+        cover.DOMAIN,
+        {
+            cover.DOMAIN: {
+                "platform": "mqtt",
+                "name": "test",
+                "command_topic": "command-topic",
+                "qos": 0,
+                "payload_open": "OPEN",
+                "payload_close": "CLOSE",
+                "payload_stop": None,
+            }
+        },
+    )
+
+    assert hass.states.get("cover.test").attributes["supported_features"] == 3
+
+
 async def test_with_command_topic_and_tilt(hass, mqtt_mock):
     """Test with command topic and tilt config."""
     assert await async_setup_component(