USB hub: make USB device id constant

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>
Signed-off-by: NNémeth Márton <nm127@freemail.hu>
Cc: Julia Lawall <julia@diku.dk>
Cc: cocci@diku.dk
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/hub.c

@@ -3432,7 +3432,7 @@ static int hub_thread(void *__unused)
 	return 0;
 }
 
-static struct usb_device_id hub_id_table [] = {
+static const struct usb_device_id hub_id_table[] = {
     { .match_flags = USB_DEVICE_ID_MATCH_DEV_CLASS,
       .bDeviceClass = USB_CLASS_HUB},
     { .match_flags = USB_DEVICE_ID_MATCH_INT_CLASS,