set correct minimum dpi for AV_ASIC_C6 on avision

I wonder, if you would mind integrating this patch into the main branch. For a hp scanjet 8250, a minimum resolution of 50dpi does not work and produces nonsense results (unfortunately xsane seems to default to this resolution for the preview window), with this change for this scanner everything works fine. This is a very simple change, that sets the minimum resolution for all scanners with AV_ASIC_C6 to this minimum resolution. If you are opposed to that, I could try to rework the patch to only set the resolution for this scanner. Not sure, why the scanner support is listed as good at the current state, this issue is clearly re-producible for me?