thermal/drivers/int3400: Use thermal zone device wrappers

Use the thermal core API to access the thermal zone "type" field
instead of directly using the structure field.  While here, remove
access to the temperature field, as this driver is reporting fake
temperature, which can be replaced with INT3400_FAKE_TEMP. Also
replace hardcoded 20C with INT3400_FAKE_TEMP

Acked-by: srinivas pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>