UBI: fix resource de-allocation

GregKH asked to fix UBI which has fake device release method. Indeed,
we have to free UBI device description object from the release method,
because otherwise we'll oops is someone opens a UBI device sysfs file,
then the device is removed, and he reads the file. With this fix, he
will get -ENODEV instead of an oops.

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