staging: comedi: addi_apci_1564: remove apci1564_do_config

The DO config function served the purpose of configuring the diagnostic
interrupts for the board.  As the driver currently does not support
diagnostic interrupts, the digital output subdevice does not need an
insn_config operation and this function can be safely removed.

Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>