thermal: cpu_cooling: Clarify error message

Make it clear that it is a failure if the cpufreq driver was unable to
register as a cooling device. Makes it easier to find in logs and
grepping for words like fail, err, warn.

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