staging: comedi: ni_660x: tidy up ni_660x_set_pfi_routing()

Use the comedi.h provided constants (enum ni_660x_pfi_routing) instead
of defining new ones for the output sources.

Use a switch to clarify the channel/source validation.

For aesthetics, rename the private data members 'pfi_output_selects' and
'pfi_direction_bits'.

Remove the 'min_counter_pfi_chan' and 'max_dio_pfi_chan' from
enum ni_660x_constants. The open coded values make the code easier
to follow.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>