thermal: thermal_hwmon: Fix a kernel NULL pointer dereference

When the hwmon device node of a thermal zone device is not found,
using hwmon->device causes a kernel NULL pointer dereference.

Fixes: dec07d39 ("thermal: Don't use 'device' internal thermal zone structure field")
Reported-by: Preble Adam C <adam.c.preble@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>