Add appropriate `const` modifiers to member functions of the shared memory cache

[weinbe2]

Many of the member functions of the shared memory cache helper ( https://github.com/lattice/quda/blob/develop/include/targets/cuda/shared_memory_cache_helper.h ) lack const modifiers, which leads to compiler errors when a shared memory cache is passed to a function via a constant reference.

This isn't a blocking issue as it's easy to work around, but it would be a good quality-of-life/best practices update nonetheless.