Reset mStopPolling to false when we call PrepareForTransfer to start …

…polling again

- This covers the use case where we have an image available but the requestor rejects
  the transfer for some reason. On the next query image the requestor is able to
  continue with the BDX transfer. The provider state was not being reset for this scenario
  due to the polling flag not being reset.

- Fixes project-chip#23573

src/protocols/bdx/TransferFacilitator.cpp

@@ -102,6 +102,7 @@ CHIP_ERROR Responder::PrepareForTransfer(System::Layer * layer, TransferRole rol
 
     mPollFreq    = pollFreq;
     mSystemLayer = layer;
+    mStopPolling = false;
 
     ReturnErrorOnFailure(mTransfer.WaitForTransfer(role, xferControlOpts, maxBlockSize, timeout));