thermal: gov_bang_bang: Clean up thermal_zone_trip_update()

thermal: gov_bang_bang: Clean up thermal_zone_trip_update()

Do the following cleanups in thermal_zone_trip_update():

 * Drop the useless "zero hysteresis" message.
 * Eliminate the trip_index local variable that is redundant.
 * Drop 2 comments that are not useful.
 * Downgrade a diagnostic message from pr_warn() to pr_debug().
 * Use consistent field formatting in diagnostic messages.

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>