A few cleanups & fixes to the epjitsu backend

This rebases !30 (closed) by @marschap on top of latest master. I could not push to the original branch.

Original description:

This merge request has a few cleanups & fixes for the epjitsu driver:

it introduces the MAX() & MIN() functions to allow easier understanding of the code it fixes a typo adds DBG() instrumentation to some functions / fixes it makes sure that some ..._range.max values are larger than their ....range.min counterparts fixes an issue with ADF scanners where the page width was ignored

Please consider merging them into master Thanks in advance Peter