staging: comedi: das1800: remove 'do_n_chan' boardinfo

The "hc" type boards have 8 digtial outputs. All other board
types have 4.

Remove the 'do_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>