kingst-la2016: reduce a little if-deffery in logic threshold support

kingst-la2016: reduce a little if-deffery in logic threshold support

Move a logic threshold related switch case to the location where its
sibling config keys reside. Which eliminates a separate distant chunk
under an ifdef which should not have been there in the first place.

Make the presence of a device context member depend on the choice of
simple or complex threshold support. The simple approach exclusively
requires one index into the discrete set of a few voltages.