net: lwm2m: Fix device obj missing error code ri

From the OMA LwM2M Object and Resource Registry: "When the single Device
Object Instance is initiated, there is only one error code Resource
Instance whose value is equal to 0 that means no error."

This fix creates that initial error code resource instance, and makes
sure that it doesn't get deleted by the Reset Error Code resource.

Signed-off-by: Tjerand Bjornsen <tjerand.bjornsen@nordicsemi.no>