thermal: of-thermal: Reduce log level for message when can't fine thermal zone

Some systems register thermal zone by themself and don't need to
have thermal zones node in DT. Therefore reduce the log level from
ERROR to DEBUG when thermal zone node can't be find in
of_thermal_destroy_zones().

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>