staging: comedi: ke_counter: add digital output support

The hardware has three digital output channels. Add the subdevice
support for these channels.

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>