mtd: docg3: fix 'defined but not used' warning

With CONFIG_OF=n, we can see the following warning:

   drivers/mtd/devices/docg3.c:2122:28: warning: 'docg3_dt_ids' defined but not used [-Wunused-variable]
    static struct of_device_id docg3_dt_ids[] = {

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>