staging: comedi: ii_pci20kc.c: remove 'iobase' from the subdevice private data

The 'iobase' can be calculated when needed. Remove it from the
subdevice private data.

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