Remove unused signals, renaming.

And refactoring.

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>

This bug was not affecting the design, as it would read once from an empty FIFO only after
the transfer was done.

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

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

This commit fixes an issue that would cause the L2P DMA to drop some data when the WB slave would
stall at the same cycle as the dual-clock FIFO would raise the 'full' flag.

The WB state machine has been redesigned in order to solve this issue and make sure that data is
properly retained when this condition appears.

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

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

Remove the option to use the 200MHz PCI clock for the complete DMA engine to avoid compicating the design
and introducing too many alternatives that will need to be tested, now and in the future.

On the SPEC, it has been shown that with the latest modifications it is trivial to meet timing when using
a 125MHz (asynchronous to the PCI) clock for DMA.

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

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

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

Note: P2L DMA already supported this for DMA writes.