[Stepper] linear mode requires control for every step

[loganmzz]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When using linear stepper, I don't want to provide a control.

What is the current behavior?

You can't switch to a step without control when linear mode is activated.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-rtrzvu

What is the use-case or motivation for changing an existing behavior?

Makes linear usage more smooth without need to add unused code and objects.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular Material 2.0.0-beta 12

Is there anything else we should know?

[angular-robot]

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[angular-robot]

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