thermal/core: Move thermal_set_delay_jiffies to static

The function 'thermal_set_delay_jiffies' is only used in
thermal_core.c but it is defined and implemented in a separate
file. Move the function to thermal_core.c and make it static.

Cc: Alexandre Bailon <abailon@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linexp.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lore.kernel.org/r/20220722200007.1839356-7-daniel.lezcano@linexp.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>