Add missing return statements in setters

[niccifor]

Description

There were 2 missing return statements in
Magento\Sales\Model\Order\Shipment\ItemCreation setters.

This brakes "fluent interface" suggested in its interface.

magento2/app/code/Magento/Sales/Api/Data/LineItemInterface.php

Line 30 in fa2439c

* @return $this

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)