staging: comedi: addi_common.h: remove subdevice callbacks from struct addi_board

None of the drivers that still include this header use the subdevice callbacks.
Remove all of them to reduce the bloat.

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>