thermal/drivers/db8500: Use driver dev instead of tz->device

The db8500 driver uses the thermal zone device instead of the device
attached to it. In order to prevent the drivers to access the thermal
zone device structure, replace the thermal zone device by the driver
to show the debug message.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230307133735.90772-5-daniel.lezcano@linaro.org