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

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>
(cherry picked from commit 73b9bcd76d13716cc0e0ab053f8c1ae41f47636e)
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Conflicts:
	drivers/thermal/cpu_cooling.c