Use speaker-multiple icon for NAD receiver (#34572)

By default media players get a chrome cast icon, which feels
quite wrong for my old style amplifier that doesn't have any
connection to the rest of the word, except for a RS232 port.

homeassistant/components/nad/media_player.py

@@ -129,6 +129,11 @@ class NAD(MediaPlayerEntity):
         """Return the state of the device."""
         return self._state
 
+    @property
+    def icon(self):
+        """Return the icon for the device."""
+        return "mdi:speaker-multiple"
+
     @property
     def volume_level(self):
         """Volume level of the media player (0..1)."""