genesys: Improve gray scan quality on LiDE 220
The fix has been suggested by Matthew Petroff matthew@mpetroff.net.
The MR replaces !379 (closed) which implemented the same thing in a little bit less clean way.
Fixes #106 (closed), fixes #52 (closed).
Thanks @mpetroff for suggesting the initial fix.
Edited by Povilas Kanapickas