thermal: core: remove unnecessary mutex_init()

The mutex poweroff_lock is initialized statically. It is
unnecessary to initialize by mutex_init().

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200916062139.191233-1-miaoqinglang@huawei.com