surface: Ensure min_position < max_position in _get_min_max_positions

surface: Ensure min_position < max_position in _get_min_max_positions

Otherwise log the configuration and raise a ValueError.

It's unlikely that a user misconfigures their ranges, but it is
possible.  It is also possible that a bug in pycomedi---like the one
fixed by 27562b0 (channel: Fix to-physical setup in
AnalogChannel._get_converter, 2013-01-17)---leads to a broken
inverse-conversion-coefficients configuration.  That bug is now fixed,
but if it happened once, it might happen again ;).