staging: comedi: pcmuio: remove 'iobase' from private data

It's simple enough to calculate the iobase when needed. Remove this
member from the private data.

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