thermal: cpu_cooling: fix return value check in cpufreq_cooling_register()

In case of error, the function thermal_cooling_device_register() returns
ERR_PTR() and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>