staging: comedi: cb_das16_cs: fix analog output subdevice init

The analog output subdevice is only available on the -AO version
of the DAS16/16 device and the number of channels is provided in
the boardinfo. Make sure the subdevice is marked as unused for
devices that do not support the analog out.

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