thermal: exynos_thermal: return a proper error code while thermal_zone_device_register fail.

we are returning EINVAL while the thermal_zone_device_register function fail.
instead we can use the return value from the thermal_zone_device_register by
using PTR_ERR.

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