thermal: cpu_cooling: return ERR_PTR() for !CPU_THERMAL or !THERMAL_OF

The documentation of of_cpufreq_cooling_register() and
cpufreq_cooling_register() say that they return ERR_PTR() on error.
Accordingly, callers only check for IS_ERR().  Therefore, make them
return ERR_PTR(-ENOSYS) as is customary in the kernel when config
options are missing.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>