Fix ignored return value in example_12

[habartakh]

When building the package ros2_control_demo_example_12, the following warning is generated:

 /ros2_ws/src/ros-controls/ros2_control_demos/example_12/controllers/src/passthrough_controller.cpp:145:39: warning: ignoring return value of ‘bool hardware_interface::LoanedCommandInterface::set_value(const T&, unsigned int) [with T = double]’, declared with attribute ‘nodiscard’ [-Wunused-result]
  145 |       command_interfaces_[i].set_value(reference_interfaces_[i]);
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~

This was fixed by explicitly ignoring the return value of the set_value function.

[christophfroehlich]

Shouldn't we use this return value here, for exemplary purposes? For example, giving a warning if false? Using RCLCPP_INFO_EXPRESSION maybe.