genesys: Fix completely broken scanning on GL646

Recent changes in motor code exposed some-preexisting bugs and caused regressions. !274 (merged) caused the motor tables to be computed in a better way which exposed completely broken backtracking calculation. !275 (merged) exposed wrong logic that governs motor stopping. Additionally, a refactoring in !275 (merged) missed start_scan() in gl646 move_back_home() function, which now updates scan head position and threw exception due too long feeding distance.