staging: comedi: pcmmio: simplify pcmmio_dio_insn_config()

Currently this function uses the subdevice private data to get the
iobase address needed to update the channel configuration. This
subdevice private data is in the process of being removed.

Use the subdevice 'index' to determine the base 'port' needed to
access the correct digital i/o registers. The pcmmio_dio_write()
function can then be used to update the configuration.

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>