dbdma: Clear cmd_in_progress before callback.

Because the callback might start DMA commands.

devices/common/dbdma.cpp

@@ -467,11 +467,11 @@ void DMAChannel::start() {
 
     this->queue_len = 0;
 
+    this->cmd_in_progress = false;
+
     if (this->start_cb)
         this->start_cb();
 
-    this->cmd_in_progress = false;
-
     // some DBDMA programs contain commands that don't transfer data
     // between a device and memory (LOAD_QUAD, STORE_QUAD, NOP and STOP).
     // We thus interprete the DBDMA program until a data transfer between