staging: comedi: cb_pcidda: remove private data variable 'dac'

The private data variable 'dac' is used to hold the PCI base
address for the board. Use the 'iobase' variable provided by
the comedi_device instead.

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>