staging: comedi: pcmmio: remove 'iobase' from subdevice private data

The 'iobase' is only used to pass the iobase address of the analog output
registers to ao_winsn(). This address is simply dev->iobase + 8.

Use that instead and remove the 'iobase' from the subdevice private data.
This removes a couple more uses of the ugly 'subpriv' macro.

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>