thermal/idle_inject: Fix comment of idle_duration_us and name of latency_ns

The comment of idle_duration_us and the name of latency_ns can be misleading,
so fix them.

Signed-off-by: zhuguangqing <zhuguangqing@xiaomi.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200917073553.898-1-zhuguangqing83@gmail.com