usb: gadget: constify usb_gadget_ops structures

Declare usb_gadget_ops structures as const as they are only stored in
the ops field of a usb_gadget structure. This field is of type const, so
usb_gadget_ops structures having this property can be declared const
too.
Done using Coccinelle:

@r1 disable optional_qualifier@
identifier i;
position p;
@@
static struct usb_gadget_ops i@p={...};

@ok1@
identifier r1.i;
position p;
struct fotg210_udc fotg210;
@@
fotg210.gadget.ops=&i@p

@bad@
position p!={r1.p,ok1.p};
identifier r1.i;
@@
i@p

@depends on !bad disable optional_qualifier@
identifier r1.i;
@@
+const
struct usb_gadget_ops i;

File size before:
   text	   data	    bss	    dec	    hex	filename
   7559	    384	      8	   7951	   1f0f	usb/gadget/udc/fotg210-udc.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   7655	    288	      8	   7951	   1f0f	usb/gadget/udc/fotg210-udc.o
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/udc/fotg210-udc.c

@@ -1058,7 +1058,7 @@ static int fotg210_udc_stop(struct usb_gadget *g)
 	return 0;
 }
 
-static struct usb_gadget_ops fotg210_gadget_ops = {
+static const struct usb_gadget_ops fotg210_gadget_ops = {
 	.udc_start		= fotg210_udc_start,
 	.udc_stop		= fotg210_udc_stop,
 };