backend/avision: fix lock-up of scanners not supporting fastfeed
de19ebc6 introduced a regression, where some scanners would lock-up completely and require a full power cycle.
To fix that, do not call release_unit for any scanners not supporting fastfeed for now, since that leads to lock-ups on at least one device. Currently, it isn't clear what exactly is causing the problems, so it maybe be enabled in a correct way again, later.
Resolves issue #337 (closed).
Signed-off-by: Michael Niewöhner foss@mniewoehner.de
Edited by Olaf Meeuwissen