staging: comedi: addi_apci_1500: remove private data 'iobase'

This member of the private data is the same as the comedi_device 'iobase'.
Remove the private data member and use that instead.

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>