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/s3c-hsotg.c b/drivers/usb/gadget/s3c-hsotg.c
index 833d85b..26033b5 100644
--- a/drivers/usb/gadget/s3c-hsotg.c
+++ b/drivers/usb/gadget/s3c-hsotg.c
@@ -3055,7 +3055,7 @@
 	return 0;
 }
 
-static struct usb_gadget_ops s3c_hsotg_gadget_ops = {
+static const struct usb_gadget_ops s3c_hsotg_gadget_ops = {
 	.get_frame	= s3c_hsotg_gadget_getframe,
 	.udc_start		= s3c_hsotg_udc_start,
 	.udc_stop		= s3c_hsotg_udc_stop,