thermal/drivers/rockchip: Simplify channel id logic

Replace the channel ID lookup table by a simple offset, since
the channel IDs are consecutive.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20230308112253.15659-5-sebastian.reichel@collabora.com