ruby: fix issue with unused var in DMASequencer

the iterator declared in DMASequencer::ackCallback() is only used in an
assert, this causes clang to fail when building fast. here we move
the find call on the request table directly into the assert.

src/mem/ruby/system/DMASequencer.cc

@@ -191,9 +191,7 @@ DMASequencer::dataCallback(const DataBlock & dblk, const Addr& address)
 void
 DMASequencer::ackCallback(const Addr& address)
 {
-    RequestTable::iterator i = m_RequestTable.find(address);
-    assert(i != m_RequestTable.end());
-
+    assert(m_RequestTable.find(addres) != m_RequestTable.end());
     issueNext(address);
 }