staging: comedi: ni_660x: clean up pfi routing

Cleans up the pfi routing code to make it easier to follow, read, and also
to prepare to use this cleaned up code for enabling the device-global
routing interface for ni_660x devices.

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>