staging: comedi: add new device-global config interface

Adds interface for configuring options that are global to all sub-devices.
For now, only options to configure device-globally identified signal routes
have been defined.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>