platform/chrome: cros_ec_chardev: Consume cros_ec_device via revocable

The cros_ec_chardev driver provides a character device interface to the
ChromeOS EC.  A file handle to this device can remain open in userspace
even if the underlying EC device is removed.

This creates a classic use-after-free vulnerability.  Any file operation
(ioctl, release, etc.) on the open handle after the EC device has gone
would access a stale pointer, leading to a system crash.

To prevent this, leverage the revocable and convert cros_ec_chardev to a
resource consumer of cros_ec_device.

---
v9:
- New to the series.
- Change revocable API usages accordingly.

v4 - v8:
- Doesn't exist.

v3: https://lore.kernel.org/all/20250912081718.3827390-6-tzungbi@kernel.org
- Use specific labels for different cleanup in cros_ec_chardev_open().

v2: https://lore.kernel.org/all/20250820081645.847919-6-tzungbi@kernel.org
- Rename "ref_proxy" -> "revocable".
- Fix a sparse warning by removing the redundant __rcu annotation.

v1: https://lore.kernel.org/all/20250814091020.1302888-4-tzungbi@kernel.org

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