thermal: Use thermal_zone_device_type() accessor

Replace the accesses to 'tz->type' by its accessor version in order to
self-encapsulate the thermal_zone_device structure.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com> #mlxsw
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> #MediaTek LVTS
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>