staging: comedi: daqboard2000: remove unused 'card' member

The `card` member of `struct daqboard2000_private` and the enumerated
constant `card_daqboard_2000` are not used.  Remove them.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>