isdn: hisax: hfc_usb: 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: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/isdn/hisax/hfc_usb.c

@@ -65,7 +65,7 @@ typedef struct {
 } hfcsusb_vdata;
 
 /* VID/PID device list */
-static struct usb_device_id hfcusb_idtab[] = {
+static const struct usb_device_id hfcusb_idtab[] = {
 	{
 		USB_DEVICE(0x0959, 0x2bd0),
 		.driver_info = (unsigned long) &((hfcsusb_vdata)