usb: gadget: pxa27x_udc: fix warning in non device-tree build

The recent change bringing device-tree support triggers a warning in a
non device-tree build :
  drivers/usb/gadget/udc/pxa27x_udc.c:2405:28: warning: 'udc_pxa_dt_ids'
  defined but not used [-Wunused-variable]

Fix the warning with a preprocessor condition.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/udc/pxa27x_udc.c

@@ -2400,11 +2400,13 @@ static struct pxa_udc memory = {
 	}
 };
 
+#if defined(CONFIG_OF)
 static struct of_device_id udc_pxa_dt_ids[] = {
 	{ .compatible = "marvell,pxa270-udc" },
 	{}
 };
 MODULE_DEVICE_TABLE(of, udc_pxa_dt_ids);
+#endif
 
 /**
  * pxa_udc_probe - probes the udc device