mtd: ubi: Restore missing cleanup on ubi_init() failure path

We need to clean-up debugfs and ubiblock if we fail after initialising
them.

Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Fixes: 927c1452 ("mtd: ubi: attach from device tree")
Signed-off-by: Richard Weinberger <richard@nod.at>