ps2: correctly handle 'get/set scancode' command

When getting scancode, current scancode must be preceded from reply ack.
When setting scancode, we must reject invalid scancodes.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Message-id: 1473969987-5890-3-git-send-email-hpoussin@reactos.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>