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/renesas_usbhs/mod_gadget.c b/drivers/usb/renesas_usbhs/mod_gadget.c
index dd41f61..8097450 100644
--- a/drivers/usb/renesas_usbhs/mod_gadget.c
+++ b/drivers/usb/renesas_usbhs/mod_gadget.c
@@ -905,7 +905,7 @@
 	return 0;
 }
 
-static struct usb_gadget_ops usbhsg_gadget_ops = {
+static const struct usb_gadget_ops usbhsg_gadget_ops = {
 	.get_frame		= usbhsg_get_frame,
 	.set_selfpowered	= usbhsg_set_selfpowered,
 	.udc_start		= usbhsg_gadget_start,