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/m66592-udc.c b/drivers/usb/gadget/m66592-udc.c
index 0a35db1..34a0cb9 100644
--- a/drivers/usb/gadget/m66592-udc.c
+++ b/drivers/usb/gadget/m66592-udc.c
@@ -1526,7 +1526,7 @@
 	return 0;
 }
 
-static struct usb_gadget_ops m66592_gadget_ops = {
+static const struct usb_gadget_ops m66592_gadget_ops = {
 	.get_frame		= m66592_get_frame,
 	.udc_start		= m66592_udc_start,
 	.udc_stop		= m66592_udc_stop,