axi_demux: Replace write FIFO (w_fifo) with a write credit counter

[thommythomaso]

No description provided.

[WRoenninger]

From #116:

axi_demux: Remove FIFO between AW and W channel, is now a register plus counter.
Prevents AWs to be emmitted downstream to a different master port as long as Ws
are still in flight to another. This prevents deadlock, if there is stalling
downstream.