staging: comedi: cb_pcidas: cleanup the boardinfo 3

Remove the ai_diff_chans variable. This option is not used in
the driver. If it is used later, the value can be easily found
by using ai_se_chans/2.

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>