Undo unintentional change to deco.c

In commit d163a68ac69e "Clean up the rewritten deco.c" I apparently made
one more change than I intended - I changed the last deco stop back to 3m
instead of allowing the smooth mode to go all the way back to 0 without
any discrete steps.

This fixes that mistake.

Reported-by: Robert C. Helling <helling@lmu.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>