UBI: Fix error path in scan_pool()

We have to set "ret", not "err" in case of an error.

Reported-and-tested-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>