mtd: docg3: fix 'defined but not used' warning

With CONFIG_OF=n, we can see the following warning:

   drivers/mtd/devices/docg3.c:2122:28: warning: 'docg3_dt_ids' defined but not used [-Wunused-variable]
    static struct of_device_id docg3_dt_ids[] = {
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>

drivers/mtd/devices/docg3.c

@@ -2119,11 +2119,13 @@ static int __exit docg3_release(struct platform_device *pdev)
 	return 0;
 }
 
+#ifdef CONFIG_OF
 static struct of_device_id docg3_dt_ids[] = {
 	{ .compatible = "m-systems,diskonchip-g3" },
 	{}
 };
 MODULE_DEVICE_TABLE(of, docg3_dt_ids);
+#endif
 
 static struct platform_driver g3_driver = {
 	.driver		= {