thermal/drivers/exynos: Handle devm_regulator_get_optional return value correctly

Currently, if regulator is required in the SoC, but
devm_regulator_get_optional fails for whatever reason, the execution
will proceed without propagating the error. Meanwhile there is no
reason to output the error in case of -ENODEV.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Mateusz Majewski <m.majewski2@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231201095625.301884-5-m.majewski2@samsung.com