watchdog: pika_wdt: add __ro_after_init to ident

The object ident of type watchdog_info structure is not
modified after getting initialized by pikawdt_init. Apart from getting
referenced in init it is also passed as an argument to the function
copy_to_user but this argument is of type const void *. Therefore add
__ro_after_init to its declaration.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>