staging: comedi: ni_660x: refactor GPCT_OFFSET

This driver supports boards that have 1 or 2 TIO chips with base
addresses 0x800 apart. Replace the static const array 'GPCT_OFFSET'
with a define and calculate the base address based on the chip index.

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>