thermal: cpu_cooling: fix kernel doc for is_cpufreq_valid

Update documentation for is_cpufreq_valid function so
that kernel-doc does not complain about return value.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>