thermal: core: Mark thermal zones as exiting before unregistration

thermal: core: Mark thermal zones as exiting before unregistration

In analogy with a previous change in the thermal zone registration code
path, to ensure that __thermal_zone_device_update() will return early
for thermal zones that are going away, introduce a thermal zone state
flag representing the "exit" state and set it while deleting the thermal
zone from thermal_tz_list.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/4394176.ejJDZkT8p0@rjwysocki.net
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>