staging: comedi: adv_pci1723: tidy up pci1723_dio_insn_config()

For aesthetics, tidy this function up and use the register defines
to remove the magic numbers.

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>