thermal: exynos: check return values of ->get_trip_[temp, hyst] methods

Check return values of ->get_trip_[temp,hyst] methods in
exynos_tmu_initialize().

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>