staging: comedi: das1800: initialize all analog outputs

The current code used to initialize the analog outputs only sets the
last channel. The other channels will be reloaded with the last value
that was written to them.

Move the code into the subdevice init and properly initialize all the
channels to 0V.

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>