ubi: wl: Fix uninitialized variable

There is a potential execution path in which variable *err*
is compared against UBI_IO_BITFLIPS without being properly
initialized previously.

Fix this by initializing variable *err* to 0.

Addresses-Coverity-ID: 1477298 "(Uninitialized scalar variable")
Fixes: 663586c0 ("ubi: Expose the bitrot interface")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Richard Weinberger <richard@nod.at>