backend/genesys_gl841: Reduce extra dark area offset for Lide 80

When looking into fixing some bugs for Canoscan Lide 80, I noticed that the extra offset applied to Lide 80 scanners specifically (see https://gitlab.com/sane-project/backends/blob/master/backend/genesys_gl841.c#L4363) advances almost completely past the dark area on my scanner (see attached black_white_shading_140.pnm file). I've reduced the offset slightly so that there's a little bit of safe area to take into account the manufacturing variation (see attached black_white_shading_70.pnm).