Flush serial port before closing it.
Flush serial port before closing it.

We should flush the serial port before closing it since it will be closed imediatally,
losing whatever was pending to write.

Change-Id: I3ccb40cfb8c9f1c28845b0fb461336a5f6a83cd4
Fixes: https://tree.taiga.io/project/stoq/task/1425

stoq/stoqdrivers,stoq/stoqdrivers