thermal/core: Use the thermal zone 'devdata' accessor in hwmon located drivers

The thermal zone device structure is exposed to the different drivers
and obviously they access the internals while that should be
restricted to the core thermal code.

In order to self-encapsulate the thermal core code, we need to prevent
the drivers accessing directly the thermal zone structure and provide
accessor functions to deal with.

Use the devdata accessor introduced in the previous patch.

No functional changes intended.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Guenter Roeck <linux@roeck-us.net> #hwmon
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>