thermal: cpufreq_cooling: Check the policy first in cpufreq_cooling_register()

Since the policy needs to be accessed first when obtaining cpu devices,
first check whether the policy is legal before this.

Fixes: 5130802d ("thermal: cpu_cooling: Switch to QoS requests for freq limits")
Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>