platform/chrome: cros_ec: Call interrupt bottom half at probe time

While the AP was powered off, the EC may have send messages.
If the message is not serviced within 3s, the EC stops sending message.
Unlock the EC by purging stale messages at probe time.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210122054637.1422289-3-gwendal@chromium.org