staging: comedi: addi_apci_3xxx: properly set the analog input subdev_flags

Some of the boards supported by this driver only have differential
analog inputs. Add a new boardinfo, 'ai_subdev_flags', to allow
properly setting the subdev_flags for the analog input subdevice.

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>