thermal: imx8mm: Disable the clock on probe failure

Prior to returning an error in probe, disable the previously
enabled clock.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201202232448.2692-2-festevam@gmail.com