channel: Fix to-physical setup in AnalogChannel._get_converter

channel: Fix to-physical setup in AnalogChannel._get_converter

This fixes a bug introduced by

  commit ac38a7987800c3982ac363af5b7780885c88d3a1
  Author: W. Trevor King <wking@tremily.us>
  Date:   Fri Oct 19 09:53:22 2012 -0400

    calibration: support to-physical-only softcal boards.

where the converter's _from_physical was only initialized if there was
an error trying to determine its value.  Obviously, we should only be
initializing it if there *wasn't* an error.