avision: Make scanning multiple sheets work for AV220D2+

When scanning a larger number of pages/sheets (e.g.20) I always had to scan only one sheet (2 pages) at a time in xsane or simple-scan. This works perfectly fine, you just have to push the button to scan two more pages every second or so, after the previous two pages complete.

If you try to scan 20 pages (10 sheets) in one row, only the first two pages will be scanned. The second sheet will be fed through the scanner but then everything will lock up. The image data from the second sheet will not show up, and after some seconds, SANE will report I/O errors.

This patch makes scanning any number of pages/sheets in one go work.

Closes #453