net: lwm2m: Clear acknowledged flag before sending separate response

When using a separate response mechanism in LwM2M client, the
acknowledged flag is used to notify the engine that the request has been
acknowledged already with an empty ack and a separate CON response
should be sent. The same flag however is used by the retransmission
mechanism, to check if the CON request sent by the client has been
acknowledged by the peer.

As separate responses use the flag both ways, it has to be cleared
before sending the separate CON reply. Otherwise, the retransmission
logic assumes the reply has already been acknowledged and skips the
retransmission.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>