UBI: block: Fix error path on alloc_workqueue failure

Otherwise we'd return a random value if allocation of the workqueue fails.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>