Staging: comedi: fix bug with invalid minor number usage.

Return error instead of segfaulting if user trys to run comedi_config on
a device file with and invalid minor number.

From: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: David Schleef <ds@schleef.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>