watchdog: mei_wdt: constify mei_cl_device_id

mei_cl_device_id are not supposed to change at runtime. All functions
working with mei_cl_device_id provided by <linux/mei_cl_bus.h> work
with const mei_cl_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>