staging: comedi: addi_apci_3xxx: remove (*insn_*) callbacks from boardinfo

The same subdevice callbacks are used by all the boards supported by
this driver. Remove the information from the boardinfo and initialize
the subdevices directly.

Remove the callback initializations that are always = NULL.
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>