ps2: do not generate invalid key codes for unknown keys

Instead, print a warning message.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Message-id: 1473969987-5890-6-git-send-email-hpoussin@reactos.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
