platform/chrome: cros_ec: fix compilation warning

When including cros_ec.h solely, the compiler emits the following
warning:
> 'struct cros_ec_device' declared inside parameter list
  will not be visible outside of this definition or declaration

Fix it by forward declaration.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20231003080453.4011301-1-tzungbi@kernel.org


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