thermal/drivers/ti: Use fixed update interval

Currently the TI thermal driver sets the sensor update interval based
on the polling of the thermal zone. In order to get the polling rate,
the code inspects the thermal zone device structure internals, thus
breaking the self-encapsulation of the thermal framework core
framework.

On the other side, we see the common polling rates set in the device
tree for the platforms using this driver are 500 or 1000 ms.

Setting the polling rate to 250 ms would be far enough to cover the
combination we found in the device tree.

Instead of accessing the thermal zone device structure polling rate,
let's use a common update interval of 250 ms for the driver.

Cc: Keerthy <j-keerthy@ti.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Acked-by: Keerthy <j-keerthy@ti.com>
Link: https://lore.kernel.org/r/20230307133735.90772-7-daniel.lezcano@linaro.org