staging: comedi: ni_660x: tidy up the counter subdevices init

For aesthetics, add some whitespace to the subdevice init and use
a couple local variables to 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>