staging: comedi: das16: all supported board have digital outputs

All the boards supported by this driver have digital outputs and all
of then use the same function to handle the (*insn_bits) for the
subdevice.

Remove the 'do_' member from the boardinfo and always initialize the
digital output subdevice during the board attach.

Tidy up the subdevice init a bit as well as the (*insn_bits) function.
Remove SDF_READABLE from the subdev_flags. This is a pure digital output
subdevice and the outputs are not readable.

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>