Make return code external tls key match docs

Make return code external tls key match docs

In tls_ctx_use_external_private_key, the return codes were inverted
compared to what is documented in ssl_backend.h (and what can
reasonably be expected). Internally the return code is never checked,
so this did not directly result in any change of behavior.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180228135240.22945-1-joost@joostrijneveld.nl>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16577.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>