staging: comedi: addi_apci_3120: clarify AO subdevice init

Only the apci3120 board supports an analog output subdevice. The
boardinfo indicates this by setting the 'i_NbrAoChannel' value.

The range table as well as the function used for the insn_write
operation are fixed for the analog outputs. Remove this information
from the boardinfo and use it directly in the subdevice init.

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>