staging: comedi: cb_das16_cs: use #define'd io reg offsets

Instead of open coding the 'dev->iobase + n' for each io register
in the device, use the provided #define's for the offsets.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>