staging: comedi: das1800: remove 'ao_n_chan' boardinfo

The "st-da" board types have 4 analog output channels. All other
board types, with analog outputs, only have 2 channels.

Remove the 'ao_n_chan' member of the boardinfo and use the
'id' member to determine the subdevice 'n_chan'.

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>