thermal/drivers/armada: Use the thermal_zone_get_crit_temp()

The driver browses the trip point to find out the critical trip
temperature. However the function thermal_zone_get_crit_temp() does
already that, so the routine is pointless in the driver.

Use thermal_zone_get_crit_temp() instead of inspecting all the trip
points.

In addition, the hysteresis value is set to zero. A critical trip
point does not have a hysteresis.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20230118222610.186088-1-daniel.lezcano@linaro.org