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/r8a66597-udc.c b/drivers/usb/gadget/r8a66597-udc.c
index 5a80751..234838d 100644
--- a/drivers/usb/gadget/r8a66597-udc.c
+++ b/drivers/usb/gadget/r8a66597-udc.c
@@ -1812,7 +1812,7 @@
 	return 0;
 }
 
-static struct usb_gadget_ops r8a66597_gadget_ops = {
+static const struct usb_gadget_ops r8a66597_gadget_ops = {
 	.get_frame		= r8a66597_get_frame,
 	.udc_start		= r8a66597_start,
 	.udc_stop		= r8a66597_stop,