thermal: intel: pch: Fix unexpected shutdown at critical temperature

Like previous patch, the intel_pch_thermal device is not in ACPI
ThermalZone namespace, so a critical trip doesn't mean shutdown.

Override the default .critical callback to prevent surprising thermal
shutdoown.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201221172345.36976-2-kai.heng.feng@canonical.com