thermal: netlink: Rework notify API for cooling devices

In analogy with some previous thermal netlink API changes, redefine
thermal_notify_cdev_state_update(), thermal_notify_cdev_add() and
thermal_notify_cdev_delete() to take a const cdev pointer as their
first argument and let them extract the requisite information from
there by themselves.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>