platform/chrome: cros_ec_chardev: Leverage revocable for ec_dev

The `ec_dev` can be removed asynchronously.  `pdata->ec_dev_sem` is used
to ensure the synchronization before accessing `ec_dev`.

Revocable encapsulates the synchronization details.  Leverage revocable
for accessing the `ec_dev`.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>