xilinx: Fix attributes_test.ys

xilinx: Fix attributes_test.ys

This test pretty much passes by accident — the `prep` command runs
memory_collect without memory_dff first, which prevents merging read
register into the memory, and thus blocks block RAM inference for a
reason completely unrelated to the attribute.

The attribute setting didn't actually work because it was set on the
containing module instead of the actual memory.