epsonds: sane_cancel fix and dynamic ADF sizing based on DPI

This is a follow-up to my mailing list thread about the Epson ES-50:

https://alioth-lists.debian.net/pipermail/sane-devel/2022-August/039562.html

I tried to add the dynamic ADF scan size change in a way that should be compatible with all other scanners. Unfortunately I don't have any other Epson scanners to test with, so hopefully others can give this patch set a test and let me know if there are any issues with it.

I know it's probably bad form to roll two unrelated fixes in the same merge request, but the cancel issue was causing lots of errors in xsane and the fix was really simple.

This is my first code contribution to sane, so please let me know if I've broken any style / code convention rules.