staging: comedi: s626: simplify dio register access

This driver has three dio subdevices, each with 16 channels. These
subdevices use hardware registers that are spaced 0x10 bytes apart
for each "group" of 16 channels.

Create new macros to get the correct addresses for each register
based on the dio "group". Use the s->private (void *) to hold the
"group" number to use in the subdevice functions.

This allows removing struct dio_private and all its uses. It also
removes the ugly casts used to get the register address.

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