net: lwm2m: return BAD_REQUEST when object create fails

Per LwM2M spec (7.3.2.4 Operation on Object):
"If the payload (New Value) conveys an Object Instance ID in conflict
with one already present in the LwM2M Client, the complete request
MUST be rejected and a "Bad Request" error code MUST be sent back."

Let's do that.

Signed-off-by: Michael Scott <michael.scott@linaro.org>