staging: comedi: das08: Remove i8254_iobase from private data

The I/O port base address for the counter subdevice doesn't need to be
stored in the private data.  It can be calculated on the fly using the
offset in the static board data.

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