usb: gadget: constify all struct usb_gadget_ops

Add the missing 'const' keyword to all struct
usb_gadget_ops in the gadget framework.

Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/omap_udc.c b/drivers/usb/gadget/omap_udc.c
index d0c87b1..06be85c 100644
--- a/drivers/usb/gadget/omap_udc.c
+++ b/drivers/usb/gadget/omap_udc.c
@@ -1314,7 +1314,7 @@
 static int omap_udc_stop(struct usb_gadget *g,
 		struct usb_gadget_driver *driver);
 
-static struct usb_gadget_ops omap_gadget_ops = {
+static const struct usb_gadget_ops omap_gadget_ops = {
 	.get_frame		= omap_get_frame,
 	.wakeup			= omap_wakeup,
 	.set_selfpowered	= omap_set_selfpowered,