staging: comedi: addi_apci_1500: remove subdevice callbacks from boardinfo

This driver only has one boardinfo entry so the subdevice callbacks are
always the same. Remvoe the callbacks from the boardinfo and use them
directly when initializing the subdevices.

Remove all the subdevice callback initialization that would be set to
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>