UBI: Fix error path in scan_pool()

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

Change-Id: Ieb9ad2105a0047df4696fa8edf8bb25541600643
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>
Git-commit: f240dca8f2fdf11d52a7bacd867e274c3b21a7d3
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>