thermal: Store thermal mode in a dedicated enum

Prepare for storing mode in struct thermal_zone_device.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
[for acerhdf]
Acked-by: Peter Kaestle <peter@piie.net>
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/20200629122925.21729-3-andrzej.p@collabora.com