ac: Fix loading a dvec3 from an SSBO

The comment was wrong, since the loop above casts to a type with the
correct bitsize already.

Fixes: 7e7ee826 ("ac: add support for 16bit buffer loads")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 27fe3f5b)