dev: Fix OnIdle test in DmaReadFifo

OnIdle() is never called since DMA active check is completely
opposite to what it should be. old active status should be 'true'
and new active status should be false for OnIdle to be called

Change-Id: I94eca50edbe96113190837c7f6e50a0d061158a6
Reported-by: Rohit Kurup <rohit.kurup@arm.com>
Signed-off-by: Rohit Kurup <rohit.kurup@arm.com>
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3966
Reviewed-by: Michael LeBeane <Michael.Lebeane@amd.com>

src/dev/dma_device.cc

@@ -438,7 +438,7 @@ DmaReadFifo::dmaDone()
     handlePending();
     resumeFill();
 
-    if (!old_active && isActive())
+    if (old_active && !isActive())
         onIdle();
 }