staging: comedi: amplc_pci230: remove ao_chans member

The PCI230(+) has an AO subdevice with 2 channels, but the PCI260(+) has
none.

The `ao_chans` member of `struct pci230_board` indicates whether the
board has an AO subdevice and the number of AO channels.  The
`ao_bits` member indicates the AO sample width in bits and will only be
non-zero for boards with an AO subdevice.

Use `ao_bits` to indicate whether the board has an AO subdevice.  If it
has, assume the the number of AO channels is 2.  Then the `ao_chans`
member becomes redundant and can be removed.

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