ControllerContext.dataNodeChildByQName method refactoring

ControllerContext.dataNodeChildByQName() is used only in childByQName()
methods with DataNodeContainer input parameters:
- ContainerSchemaNode
- ListSchemaNode
- Module
Therefore child node of these DataNodeContainers can't be ChoiceCaseNode
(because it can be only child node of ChoiceNode)
and original branch >if (node instanceof ChoiceCaseNode)< couldn't be
executed.
I think that it was typo.

Change-Id: Ieb85ad8acaf047b47813542d8968c399a4e30b50
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>