thermal: samsung: Only update available threshold limits

Currently the threshold limits are updated in 2 stages, once for all
software trigger levels and again for hardware trip point.

While updating the software trigger levels, it overwrites the threshold
limit for hardware trip point thereby forcing the Exynos core to issue
an emergency shutdown.

Updating only the required fields in threshold register fixes this issue.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>