FFT megacore (variable-stream) sink_ready deasserts after dynamic size change 16->32

Okay, I was able to fix this problem after carefully timing events as the size changes. SOP should pulse on the same cycle as fftpts_in is changed, and on the next cycle, I de-asserted sink_valid to prevent loading of more data into FFT while the FFT is outputting results of previous size. While this is happening, sink_ready is de-asserted by the core. This can now reassert when the pipeline is empty.