platform/chrome: cros_ec_proto: drop unneeded BUG_ON() in prepare_packet()

prepare_packet() gets called if `ec_dev->proto_version` > 2.  For now, it
must be equivalent to EC_HOST_REQUEST_VERSION.

Drop the BUG_ON().

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220513044143.1045728-2-tzungbi@kernel.org