staging: comedi: adl_pci6208: remove the boardinfo 'ao_bits'

All the boards supported by this driver have 16-bit DACs. The
ao_bits variable in the boardinfo is commented out anyway.
Just 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>