IO Mixer Fix

[acfloria]

This PR provides a clean solution for the issue #7861. The Flaperons are again added to the mixer as they now do not case any harm anymore.

The problem was caused that callback would not modify the control value in case that it failed to get an control value. The fact that the input value is not initialized with a value in the mix function of the simple mixer resulted that it kept its value between iterations.

So in case of the right aileron for the AAERTWF mixer in my case the following happened (L301:310):

1. During the first iteration based on the roll input the input would be assigned for example with 0.5 by the control callback.
2. Due to the positive scale of the output it would result in sum = 5000.
3. In the next iteration the control callback would not modify the input value as the corresponding input was not defined. So the input variable has still the value 0.5.
4. Due to the negative scale of the flaperons '5000' is subtracted resulting in sum = 0.
5. Therefore the output would always be 0 regardless of the roll input.

[dagar]

I'm glad to see a fix to the underlying issue.