[Admittance Controller] Use control_interface::helpers method to sort the interfaces.

[destogl]

    Can we use here a helper method for sorting reference interfaces out is they exist?

The same one I mentioned before that is used in JTC.

Maybe we need another helper method for this, which would be also fine, since this could be something that is done very oftten. (We should open follow-up if such method is needed).

Originally posted by @destogl in https://github.com/ros-controls/ros2_controllers/pull/370#discussion_r953947980