staging: comedi: amplc_dio200: use register offsets in subdevices

Replace the absolute, modified I/O base addresses in the private data
structures for the '8254' counter and 'intr' DIO subdevices with offsets
from the main I/O base address.  `dio200_subdev_intr_init()` now needs
the offset instead of the absolute address.  `dio200_subdev_8254_init()`
now only needs the offset instead of both the absolute address and the
offset.  The '8255' DIO subdevices are unaffected as they are handled by
the external "8255" module.

This change is not useful by itself but is an intermediate step for
later changes.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>