staging: comedi: addi_apci_3xxx: remove 'i_NbrAiChannelDiff' boardinfo

The number of differential analog input channels is always half the
number of single-ended channels, 'ai_n_chan', or all the channels
if the board does not support single-ended channels. We can use the
'ai_subdev_flags' boardinfo to determine if the board does not support
single-ended channels.

To simplify the boardinfo, remove the 'i_NbrAiChannelDiff' and just
use 'ai_n_chan' to specify the number of channels.

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>