usb: musb: gadget: fix compilation warning

Fixes below compilation warning when musb driver is compiled for
PIO mode:

drivers/usb/musb/musb_gadget.c: In function 'musb_g_rx':
drivers/usb/musb/musb_gadget.c:840:
		warning: label 'exit' defined but not used
Signed-off-by: Ajay Kumar Gupta <ajay.gupta@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_gadget.c

@@ -896,7 +896,9 @@ void musb_g_rx(struct musb *musb, u8 epnum)
 		if (!request)
 			return;
 	}
+#if defined(CONFIG_USB_INVENTRA_DMA) || defined(CONFIG_USB_TUSB_OMAP_DMA)
 exit:
+#endif
 	/* Analyze request */
 	rxstate(musb, to_musb_request(request));
 }