thermal: rcar_thermal: propagate return value of thermal_zone_device_register

thermal_zone_device_register returns a value contained in the pointer itself
use PTR_ERR to obtain the address and return it at the end.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>