staging: comedi: addi_apci_3120: rename APCI3120_WRITE_MODE_SELECT

For aesthetics, rename this define used for the mode register offset.

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>