platform/chrome: cros_ec: Separate initialization from cros_ec_register()

platform/chrome: cros_ec: Separate initialization from cros_ec_register()

Move the initialization of the `struct cros_ec_device` from
cros_ec_register() into cros_ec_device_alloc().

This decouples device initialization from registration.  By doing so,
the per-device lock is now available immediately after allocation,
allowing it to be used safely even before the device is fully
registered.

Link: https://lore.kernel.org/r/20250828083601.856083-4-tzungbi@kernel.org
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>