xerox_mfp: SCX-4521 fixes are not merged

Back in 2017, Marc Schink has fixed several bugs of Samsung SCX-4521 scanner support, however they haven't been merged.
https://alioth-lists.debian.net/pipermail/sane-devel/2017-January/034987.html

I've tested the patches: all 3 still applies fine and fixes the described issues. Scanning is very slow without the patchset, as the very last segment is not received.