staging: comedi: pcmuio: cleanup DIO subdevice (*insn_{bits,config})

Use the pcmuio_{read,write}() helpers to read/write all 24 channels
instead of handling the digital I/O as three separate ports. This
simplifies both functions with minimal overhead.

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>