Add device_class safety to synology_dsm storage binary_sensors (#39310)

* add device_class: safety to storage binary_sensors

* Update binary_sensor.py

* Update homeassistant/components/synology_dsm/binary_sensor.py

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

* Update binary_sensor.py

* Import device_class Safety

from homeassistant.components.binary_sensor

* Update binary_sensor.py

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

homeassistant/components/synology_dsm/binary_sensor.py

@@ -1,7 +1,10 @@
 """Support for Synology DSM binary sensors."""
 from typing import Dict
 
-from homeassistant.components.binary_sensor import BinarySensorEntity
+from homeassistant.components.binary_sensor import (
+    DEVICE_CLASS_SAFETY,
+    BinarySensorEntity,
+)
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.const import CONF_DISKS
 from homeassistant.helpers.typing import HomeAssistantType
@@ -14,6 +17,8 @@ from .const import (
     SYNO_API,
 )
 
+DEFAULT_DEVICE_CLASS = DEVICE_CLASS_SAFETY
+
 
 async def async_setup_entry(
     hass: HomeAssistantType, entry: ConfigEntry, async_add_entities
@@ -71,3 +76,8 @@ class SynoDSMStorageBinarySensor(SynologyDSMDeviceEntity, BinarySensorEntity):
         if attr is None:
             return None
         return attr
+
+    @property
+    def device_class(self):
+        """Return the device class of this binary sensor."""
+        return DEFAULT_DEVICE_CLASS