platform/chrome: cros_ec: fix error handling in cros_ec_register()

Fix cros_ec_register() to unregister platform devices if
blocking_notifier_chain_register() fails.

Also use the single exit path to handle the platform device
unregistration.

Fixes: 42cd0ab4 ("platform/chrome: cros_ec: Query EC protocol version if EC transitions between RO/RW")
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>