Staging: comedi: fix sparse issues in comedi_bond.c

This fixes the issues raised by the sparse tool on the
comedi_bond.c file.

Cc: Calin A. Culianu <calin@ajvar.org>
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>