net: sockets: tls: Return an error on send() after session is closed

It was an overlook to return 0 on TLS send() call, after detecting that
TLS session has been closed by peer, such a behavior is only valid for
recv(). Instead, an error should be returned.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit dc52b207)