staging: comedi: mf6x4: use the comedi_device 'mmio' member

Use the new 'mmio' member in the comedi_device for the ioremap'ed
base address.

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>