platform/chrome: cros_ec_ishtp: free ishtp buffer before sending event

Recycle the ISH buffer before notifying of a response or an event. Often
a new message is sent in response to an event and in high traffic
scenarios this can lead to exhausting all available buffers. We can
ensure we are using the fewest buffers possible by freeing buffers as
soon as they are used.

Signed-off-by: Jett Rink <jettrink@chromium.org>
Signed-off-by: Mathew King <mathewk@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>