net: mqtt-sn: Remember incoming registered topic name

When a client uses wildcard subscription and a new message is
published to the matching topic for the first time, the gateway
sends REGISTER message to the client, containing the exact
topic name and a new topic ID.
This change fixes adding these topic ID and name to the internal
topics list.

Signed-off-by: Alexander Vasiliev <alexander.vasiliev@siemens.com>