thermal: gov_bang_bang: Use .trip_crossed() instead of .throttle()

The Bang-Bang governor really is only concerned about trip point
crossing, so it can use the new .trip_crossed() callback instead of
.throttle() that is not particularly suitable for it.

Modify it to do so which also takes trip hysteresis into account, so the
governor does not need to use it directly any more.

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>