staging: comedi: adl_pci6208: fix the digital i/o subdevice

The PCI-6208 board has 4 digital outputs and 4 digital inputs.

The support for the digital i/o subdevice was commented out and
the code was just cut-and-paste from the skel driver. Enable
the digital i/o subdevice by uncommenting the code and fixing
the insn_bits and insn_config functions.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>