thermal/drivers/rockchip: Simplify clock logic

By using devm_clk_get_enabled() the clock acquisition and
enabling can be done in one step with automatic error
handling.

Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20230308112253.15659-3-sebastian.reichel@collabora.com