thermal: cpu_cooling: get_level() can't fail

The frequency passed to get_level() is returned by cpu_power_to_freq()
and it is guaranteed that get_level() can't fail.

Get rid of error code.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>