net: sockets: tls: Fix TCP disconnect detection in poll()

`ztls_socket_data_check()` function ignored a fact when
`mbedtls_ssl_read()` indicated that the underlying TCP connection was
closed. Fix this by returning `-ENOTCONN` in such case, allowing
`poll()` to detect such event.

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