staging: comedi: ni_670x: removve unused 'ao_bits'

All the NI 670x boards have 16-bit anaolog output DACs. The
'ao_bits' in the boardinfo is not used so remove it.

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