mem: Remove explict cast from memhelper.
mem: Remove explict cast from memhelper.

Previously we were casting the result type to the the memory type which
is incorrect for things like dual-memory operations which still return a
single result.
(transplanted from f6343d592d16e651e6da6e5c0a074da4cf243356)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5