staging: comedi: pcmmio: simplify pcmmio_dio_insn_bits()

Currently this function uses the subdevice private data to get the
iobase address needed to update the dio output channel state and
read the current state of the input channels. 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 outputs.

Introduce a new helper function, pcmmio_dio_read(), to read the
current state of the input channels.

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>