[Touchpad KCM] Prevent save dialog if no touchpad was found

Summary:
If there is no touchpad found, the KCM still allways asked if the settings
are supposed to be saved because the widget gets changed although no options
have and can be changed. This little patch prevents it.

Test plan: With and without touchpad.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3324