thermal/drivers/exynos: Fix NULL pointer dereference when getting the critical temp

The driver is assuming the get_critical temperature exists as it is
inherited by the thermal of ops. But this one has been removed in
favor of the generic one.

Use the generic thermal_zone_get_crit_temp() function instead

Fixes: 13bea86623be ("thermal/of: Remove of_thermal_get_crit_temp(")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/thermal/samsung/exynos_tmu.c

@@ -264,9 +264,8 @@ static int exynos_tmu_initialize(struct platform_device *pdev)
 	unsigned int status;
 	int ret = 0, temp;
 
-	if (data->soc != SOC_ARCH_EXYNOS5433) /* FIXME */
-		ret = tzd->ops->get_crit_temp(tzd, &temp);
-	if (ret) {
+	ret = thermal_zone_get_crit_temp(tzd, &temp);
+	if (ret && data->soc != SOC_ARCH_EXYNOS5433) { /* FIXME */
 		dev_err(&pdev->dev,
 			"No CRITICAL trip point defined in device tree!\n");
 		goto out;