bz threshold is now 10 (instead of 15). I think it is ok for the moment.
If we manage to get higher image quality in the future, we'll change that.
max_frame is set to 350. 150 was too low => incomplete fingerprints
the driver was designed to stop the acquisition as soon as it gets a blank
frame (=> incomplete fingerprints). Now it waits to have at least 50 blank
frames before stopping.
