staging: most: hdm-usb: remove reference to read function in write only macro

This patch replaces the reference to the function show_value() with
NULL in the macro MOST_DCI_WO_ATTR used for the write only attributes.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>