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>