frontend/etherbone: Add 1-Slot PacketFIFO on LiteEthEtherboneProbe to prevent deadlock situation.

Based on @Based on @david-sawatzke's analysis:
"Otherwise, depending on the caching in the path, a deadlock might occur due to the rx path being
blocked by the tx path, being blocked by the arp request, for which the answers get dropped in the
rx path."

liteeth/frontend/etherbone.py

@@ -139,14 +139,21 @@ class LiteEthEtherboneProbe(Module):
 
         # # #
 
+        self.submodules.fifo = fifo = PacketFIFO(eth_etherbone_packet_user_description(32),
+            payload_depth = 1,
+            param_depth   = 1,
+            buffered      = False
+        )
+        self.comb += sink.connect(fifo.sink)
+
         self.submodules.fsm = fsm = FSM(reset_state="IDLE")
         fsm.act("IDLE",
-            If(sink.valid,
+            If(fifo.source.valid,
                 NextState("PROBE_RESPONSE")
             )
         )
         fsm.act("PROBE_RESPONSE",
-            sink.connect(source),
+            fifo.source.connect(source),
             source.pf.eq(0),
             source.pr.eq(1),
             If(source.valid & source.ready,