scan_adf.py: cosmetic

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Abstract: Multiple scan (ADF): Fix ADF scan:
- call sane_start() to request the next page(s)
- when sane_start() returns NO_DOCS (== rawapi raises EOFError), call sane_cancel() to end the scan

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Abstract: Multiple scan: Fix: Correctly split the pages

Signed-off-by: Jerome Flesch <jflesch@gmail.com>