Input: xen-kbdfront - make xenbus device ids constant

The ids field of the struct xenbus_device_id is constant in <linux/xen/xenbus.h>
so it makes sense to mark xenkbd_ids also constant.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>