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/fusb300_udc.c b/drivers/usb/gadget/fusb300_udc.c
index 8c2372f..9fafb00 100644
--- a/drivers/usb/gadget/fusb300_udc.c
+++ b/drivers/usb/gadget/fusb300_udc.c
@@ -1341,7 +1341,7 @@
 	return 0;
 }
 
-static struct usb_gadget_ops fusb300_gadget_ops = {
+static const struct usb_gadget_ops fusb300_gadget_ops = {
 	.pullup		= fusb300_udc_pullup,
 	.udc_start	= fusb300_udc_start,
 	.udc_stop	= fusb300_udc_stop,