USB skeleton: make USB device id constant

From: Márton Németh <[email protected]>

The id_table field of the struct usb_device_id is constant in <linux/usb.h> so it is worth to make the initialization data also constant.

The semantic match that finds this kind of pattern is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@r@
disable decl_init,const_decl_init;
identifier I1, I2, x;
@@

	struct I1 {
	  ...
	  const struct I2 *x;
	  ...
	};

@s@
identifier r.I1, y;
identifier r.x, E;
@@

	struct I1 y = {
	  .x = E,
	};

@c@
identifier r.I2;
identifier s.E;
@@

	const struct I2 E[] = ... ;

@depends on !c@
identifier r.I2;
identifier s.E;
@@
+ const

	struct I2 E[] = ...;

// </smpl>

---

diff -u -p a/drivers/usb/usb-skeleton.c b/drivers/usb/usb-skeleton.c
@@ -28,7 +28,7 @@
 #define USB_SKEL_PRODUCT_ID	0xfff0
 
 /* table of devices that work with this driver */
-static struct usb_device_id skel_table[] = {
+static const struct usb_device_id skel_table[] = {
 	{ USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
 	{ }					/* Terminating entry */
 };