staging: comedi: ni_at_ao: remove 'cfg2' from private data

This member of the private data is initialized to '0' during the
board reset then never changed. Just remove it.

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>