thermal: db8500: Use dev helper variable

The code gets easier to read like this.

Cc: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>