UBI: mark few variables as __initdata

The @mtd_devs and @mtd_dev_param variables are used only during the
initialization, and all functions that use the variables have
the __init prefix. This means we can safely mark the variables
as __initdata, which is a tiny optimization.

Impact: tiny RAM consumption optimization when UBI is used as a kernel
        module.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>