thermal: cpu_cooling: get_property() doesn't need to support GET_MAXL anymore

We don't use get_property() to find max levels anymore as it is done at boot
now. So, don't support GET_MAXL in get_property().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>