thermal_zone_of_sensor_register is documented as returning a pointer
to either a valid thermal_zone_device on success, or a corresponding
ERR_PTR() value.

In contrast, the function returns NULL when THERMAL_OF is configured
off. Fix this.
Signed-off-by: NPunit Agrawal <punit.agrawal@arm.com>
Acked-by: NGuenter Roeck <linux@roeck-us.net>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: NEduardo Valentin <edubezval@gmail.com>