Ensure DSLogic can be stopped.

Ensure DSLogic can be stopped.

After acquisition start, DSLogic stores samples in memory, and when done it
sends a USB packet with the trigger position.

This initial fillup can take some time. If the user requests a session stop
in between, the USB transfer is cancelled and the session hangs because it
is not closed properly.

This commit manages this case and closes the session properly when
acquisition is stopped by the user.

Signed-off-by: Diego F. Asanza <redacted>