thermal: armada: fix copy-paste error in armada_thermal_probe()

The return value from devm_kzalloc() is not checked correctly. The
test is done against a wrong variable. Fix it.

Fixes: e72f03ef2543 ("thermal: armada: use the resource managed registration helper alternative")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>