nixos/zigbee2mqtt: only add port to DeviceAllow if it is a device (#356573)

        # Hardening

        CapabilityBoundingSet = "";

        DeviceAllow = [

          config.services.zigbee2mqtt.settings.serial.port

        ];

        DeviceAllow = lib.optionals (lib.hasPrefix "/" cfg.settings.serial.port) [ cfg.settings.serial.port ];

        DevicePolicy = "closed";

        LockPersonality = true;

        MemoryDenyWriteExecute = false;