staging: comedi: pcmmio: define the offset to the 2nd ADC device

There are two ADC devices on this board. The first one handles ai
channels 0-7 and the second one channels 8-15.

Define the offset that is added to the iobase to access the 2nd
ADC device.

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>