staging: comedi: adv_pci1724: remove NUM_AO_CHANNELS define

This define is now only used to set each of the subdevice 'n_chan'. For
clarity, remove the define and open code the values.

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>