thermal: core: Stop polling DISABLED thermal devices

Polling DISABLED devices is not desired, as all such "disabled" devices
are meant to be handled by userspace. This patch introduces and uses
should_stop_polling() to decide whether the device should be polled or not.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200629122925.21729-10-andrzej.p@collabora.com