thermal: Fix thermal_zone_of_sensor_register to match documentation

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: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>