fix Gem5-Ramulator fails with !req_stall assertion failure

[kvprathap]

- Fixed write call back( This was not getting called).
   Since this is not called the inflight requests are never decremented after issueuing the writes.
    As a result, the numOutstanding() is always true.
- Issue retry, only when the queue is not full.

[guillemlp]

I have had the same problem and fixed similarly. Now it works. However, if you are using TLDRAM you need to modify the same function (tick) in Controller.cpp.

[SaugataGhose]

Thanks for catching this and pushing a fix! We will go through this and add changes to the TLDRAM tick() function, and merge these changes once we verify that we've addressed the problem fully.