mem: Fix Best Offset Prefetcher (BOP) learning phase code.

According to the DPC paper : "If the best score is less
than or equal to a fixed value BADSCORE, we turn prefetchoff
during the next phase"

However, the current code will turn prefetch off if the best
*offset* of the learning phase is less than BADSCORE, which
is incorrect.

Change-Id: Ib5790fe1341f2dd6328ec3b018bc52012e376723
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/46259
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@huawei.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/cache/prefetch/bop.cc

@@ -217,7 +217,7 @@ BOP::bestOffsetLearning(Addr x)
             phaseBestOffset = 0;
             resetScores();
             issuePrefetchRequests = true;
-        } else if (phaseBestOffset <= badScore) {
+        } else if (bestScore <= badScore) {
             issuePrefetchRequests = false;
         }
     }