staging: comedi: ni_660x: tidy up Digital I/O subdevice init

Add some whitespace to the Digital I/O subdevice init and add a
comment about the channels. This driver is a bit goofy, only 32 of
the 40 channels can actually be used for Digital I/Os and 32 of
them can be routed to the counters for alternate use.

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>