thermal: armada: Use PTR_ERR_OR_ZERO in armada_thermal_probe_legacy()

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>