rt2500usb: constify usb_device_id

usb_device_id are not supposed to change at runtime. All functions
working with usb_device_id provided by <linux/usb.h> work with
const usb_device_id. So mark the non-const structs as const.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ralink/rt2x00/rt2500usb.c

@@ -1911,7 +1911,7 @@ static const struct rt2x00_ops rt2500usb_ops = {
 /*
  * rt2500usb module information.
  */
-static struct usb_device_id rt2500usb_device_table[] = {
+static const struct usb_device_id rt2500usb_device_table[] = {
 	/* ASUS */
 	{ USB_DEVICE(0x0b05, 0x1706) },
 	{ USB_DEVICE(0x0b05, 0x1707) },