Staging: comedi: fix sparse issues in comedi_test.c

This fixes all issues pointed out by sparse with the comedi_test.c
file.

Cc: Joachim Wuttke <Joachim.Wuttke@icn.siemens.de>
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>