KComboBox: Return early when setting editable to previous value

Summary:
Instead of creating a new line edit, keep the previous one intact.
This ensures we don't set a new line edit for code such as the
following:

KComboBox combo(/* rw = */ true);
combo.setEditable(true);

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7967