[settings] Reject attempts to change a network device's bus ID

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdev_settings.c

 		memcpy ( netdev->ll_addr, data, len );
 		return 0;
 	}
+	if ( setting_cmp ( setting, &busid_setting ) == 0 )
+		return -ENOTSUP;
 
 	return generic_settings_store ( settings, setting, data, len );
 }