thermal: stm32: simplify the return expression of stm_thermal_prepare()

Simplify the return expression.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200921131107.93273-1-miaoqinglang@huawei.com