staging: comedi: addi_apci_1516: cleanup subdevice ops pointers

The same subdevice operations are used, as needed, for all the boards
supported by this driver. Remove the function pointers from the
boardinfo and set the subdevice operations directly in the attach.

Remove all the subdevice operations that would be set to NULL.

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>