staging: comedi: quatech_daqp_cs: ao subdevice does not support commands

The analog output subdevice does not support aynchronous commands.
Don't set the dev->write_subdev to this subdevice.

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>