platform/chrome: wilco_ec: Fix unreleased lock in event_read()

When copying an event to userspace failed, the event queue
lock was never released. This fixes that.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nick Crews <ncrews@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>