staging: comedi: amplc_dio200_common: introduce DIO200_GAT_SEL() macro

Replace the DIO200_[XYZ]GAT_SEL defines with a macro that returns the
correct register offset.

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>