Reduce multiple calls to calculate sensor unit in tplink (#124606)

accessing unit on the feature is not cached and it had
to be looked up every time

homeassistant/components/tplink/sensor.py

@@ -154,7 +154,5 @@ def _async_update_attrs(self) -> None:
 
         self._attr_native_value = value
         # Map to homeassistant units and fallback to upstream one if none found
-        if self._feature.unit is not None:
-            self._attr_native_unit_of_measurement = UNIT_MAPPING.get(
-                self._feature.unit, self._feature.unit
-            )
+        if (unit := self._feature.unit) is not None:
+            self._attr_native_unit_of_measurement = UNIT_MAPPING.get(unit, unit)