staging: comedi: adv_pci1723: tidy up DIO io_bits initialization

Tidy up the code that determines the initial io_bits (direction) of the
digital I/O subdevice and 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>