staging: comedi: addi_apci_3120: introduce apci3120_addon_write()

Introduce a helper function to handle the common code that writes a 32-bit
value to the 16-bit add-on register.

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>