thermal: remove get_mode() operation of drivers

get_mode() is now redundant, as the state is stored in struct
thermal_zone_device.

Consequently the "mode" attribute in sysfs can always be visible, because
it is always possible to get the mode from struct tzd.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
[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/20200629122925.21729-6-andrzej.p@collabora.com