Staging: comedi: rtd520: &&/|| typo

Only error out on unexpected fifo size.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Dan Christian <dac@ptolemy.arc.nasa.gov>
Cc: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>