Staging: comedi: fix build on arches that don't want comedi drivers

This should fix the problem of comedi drivers showing up on arches that
can not support the comedi subsystem (like s390).

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>