Wait for 3 empty frames before stopping the scan
The driver used to stop the scan immediately after an empty frame
(by checking for hist sum == 0), but it is possible to get empty
frames in the middle of the scan due to low finger pressure.
Waiting for 3 empty frames stop the driver wrongly aborting the scan
too early.
https://bugs.freedesktop.org/show_bug.cgi?id=56715