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

[thommythomaso]

[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.