thermal/drivers/exynos: Replace of_thermal_is_trip_valid() by thermal_zone_get_trip()

The thermal_zone_get_trip() does the same check as
of_thermal_is_trip_valid(). Replace the call to
of_thermal_is_trip_valid() by thermal_zone_get_trip().

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221003092602.1323944-10-daniel.lezcano@linaro.org