staging: comedi: ni_660x: use a local var for the 'chip_index'

Use a local variable for the 'counter->chip_index' to help shorten the
long lines and clarify the code.

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>