staging: comedi: make constants slightly more consistent

This makes INSN_CONFIG_DIO_{INPUT,OUTPUT,OPENDRAIN} simply be copies of the
already-existing constants COMEDI_INPUT, COMEDI_OUTPUT, and
COMEDI_OPENDRAIN.

This change also ensures that if a user happens to use, for example,
COMEDI_INPUT in place of where INSN_CONFIG_DIO_INPUT, the result will
always be the same and also acceptable and correct usage.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>