staging: comedi: add an 'mmio' member to comedi_device

All the comedi drivers that use memory mapped io currently have a
void __iomem * member in their private data for the driver. For
some of the drivers this is actually the only member in that data.

For convienence, add a new member to the comedi_device for this
void __iomem *.

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>