Fix snitch verilator tests#146
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Fix verilator latch warning due to partial assignment of rw obi responses.
The per-channel `busy` signal deasserted once the last AXI write beat was *issued*, not once its `B` response was received. Since a subset of Snitch API polls this `busy` signal, Snitch could proceed before the write had actually completed at its destination. Extend the condition so a channel is busy while: (i) the iDMA backend or ND midend are busy, or (ii) at least one issued AXI write is still awaiting its `B` response. A per-channel counter tracks outstanding writes and increments on each `AW` handshake and decrements on each `B` handshake.
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This PR fixes two bugs found after running verilator tests in Snitch:
obiresponses;busycondition with AXI4 write completion (receipt ofBresponse). This ensures that, once Snitch polls on thebusysignal, it will wait until the receipt of the AXI4Bresponse before launching new transactions.