thermal: core: Add and use thermal zone guard

Add and use a guard for thermal zone locking.

This allows quite a few error code paths to be simplified among
other things and brings in a noticeable code size reduction for
a good measure.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/1930069.tdWV9SEqCh@rjwysocki.net


Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>