staging: comedi: addi_apci_3120: remove boardinfo 'i_NbrAiChannel'

All boardinfo entries have this member set as '16'. This will initialize
the Analog Input 'n_chan' and cause the 'i_NbrAiChannelDiff' member to
never get used. Remove both members and initialize the Analog Input 'n_chan'
with 16 directly.

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>