dev: Add response sanity checks in PioPort

Add an assert in the PioPort that checks if a response packet from a
device has the right flags set before passing it to them rest of the
memory system.

src/dev/io_device.cc

@@ -57,7 +57,9 @@ PioPort::recvAtomic(PacketPtr pkt)
     // @todo: We need to pay for this and not just zero it out
     pkt->firstWordDelay = pkt->lastWordDelay = 0;
 
-    return pkt->isRead() ? device->read(pkt) : device->write(pkt);
+    const Tick delay(pkt->isRead() ? device->read(pkt) : device->write(pkt));
+    assert(pkt->isResponse() || pkt->isError());
+    return delay;
 }
 
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