Fold thermal_zone_trip_update() into bang_bang_control() which is the
only caller of it to reduce code size and make it easier to follow.

No functional impact.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>