Stabilization: fix bug that made autotuning unresponsive

When introducing the trimmed setpoints in #430 the default behavior became
to ignore inputs.  This is obviously inconsistent with the old code which
by default took the inputs from StabilizationDesired unless explicitly
overriden.  This restores the previous default behavior and keeps the
exceptions for weak leveling and stabilizationplus mode.
Merge pull request #684 from peabody124/stabilization_fix

Stabilization: fix bug that made autotuning unresponsive