drm: udl: 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: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/22fa8ca67a6d4a59997f463bf241ed56596fbcfa.1502526524.git.arvind.yadav.cs@gmail.com

drivers/gpu/drm/udl/udl_drv.c

@@ -111,7 +111,7 @@ static void udl_usb_disconnect(struct usb_interface *interface)
  * which is compatible with all known USB 2.0 era graphics chips and firmware,
  * but allows DisplayLink to increment those for any future incompatible chips
  */
-static struct usb_device_id id_table[] = {
+static const struct usb_device_id id_table[] = {
 	{.idVendor = 0x17e9, .bInterfaceClass = 0xff,
 	 .bInterfaceSubClass = 0x00,
 	 .bInterfaceProtocol = 0x00,