The FIFO size and full threshold need to be enough to hold if necessary all the pending read data
requests from the WB slave.

In the case of the Spartan-6 DDR controller being the WB slave, the FIFO needs to be able to store
up to 192 words (128 from the controller itself, plus 64 from our wrapper).

Since the GN4124 is used primarily on the SPEC, this is now the default value for the L2P DMA
master.

Signed-off-by: Dimitris Lampridis <dimitris.lampridis@cern.ch>