staging: comedi: addi_apci_3xxx: refactor 'i_NbrAoChannel' boardinfo

The boards that have analog outputs always have 4 channels.

Remove the 'i_NbrAoChannel' data from the boardinfo and add a bit
field to indicate that the board 'has_ao'. Use that to conditionally
initialize the subdevice and open code the s->n_chan value of 4.

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>