Staging: comedi: fix Kbuild

comedi doesn't like being built into the kernel right now, so force it
to be a module.

Reported-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>