access.direct.demultiplexer: introduce soft_flush

flush() ensures that all written data has been sent to the device.
soft_flush() on the other hand ensures that all written data has been
queued as USB URBs, but does not wait for them to complete.

Use soft_flush() to ensure that every last byte of queued data will
make it to the device in a timely manner, without stalling the entire
USB transfer pipeline.

software/glasgow/access/direct/demultiplexer.py

@@ -297,7 +297,7 @@ async def write(self, data):
                 len(self._out_tasks) < _xfers_per_queue:
             self._out_tasks.submit(self._out_task(self._out_slice()))
 
-    async def flush(self):
+    async def soft_flush(self, soft=False):
         self.logger.trace("FIFO: flush")
 
         # First, we ensure we can submit one more task. (There can be more tasks than
@@ -316,4 +316,6 @@ async def flush(self):
                 data += self._out_buffer.read()
             self._out_tasks.submit(self._out_task(data))
 
+    async def flush(self, soft=False):
+        await self.soft_flush()
         await self._out_tasks.wait_all()