genesys: no true gray with Canon LiDE 60

When requesting a true grayscale scan, i.e. not a single color component but gray values, the genesys backend will provide the Red value component nevertheless, even though the "color-filter" option is set to "None". This happens with a Canon LiDE 60 when scanning with the "mode" option set to "Gray" and the "color-filter" option set to "None".

SANE version: 1.0.27

Reproduce: Put some image on the scanner that contains red, green and blue colors. Do a "mode=Gray" scan and observe that the green and blue portions of the original come out as dark in the scan. Compare to a desaturated color scan of the same original if unsure about the observation.