Fix for broken message edit page (unknown index)

As the message module has been updated recently, the commerce_waitlist_default_message_type() function should be updated to use "message_type" instead of "message_type_email" as in the attached patch.

On the latest commerce kickstart, without this patch there is a PHP error on the message edit page for the product_available message, and product_available messages will not be sent at all.

Hope this helps someone.