usb: gadget: mv_udc: fix readl error

readl expected 'const volatile void *' as the argument.
Signed-off-by: NNeil Zhang <zhangwm@marvell.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/mv_udc_core.c

@@ -1196,7 +1196,7 @@ static int mv_udc_get_frame(struct usb_gadget *gadget)
 
 	udc = container_of(gadget, struct mv_udc, gadget);
 
-	retval = readl(udc->op_regs->frindex) & USB_FRINDEX_MASKS;
+	retval = readl(&udc->op_regs->frindex) & USB_FRINDEX_MASKS;
 
 	return retval;
 }