thermal: imx8mm: Print the correct error code

Currently the error message does not print the correct error code.

Fix it by initializing 'ret' to the proper error code.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201202232448.2692-1-festevam@gmail.com