ubi: wl: Silence uninitialized variable warning

This condition needs to be fipped around because "err" is uninitialized
when "force" is set.  The Smatch static analysis tool complains and
UBsan will also complain at runtime.

Fixes: 663586c0 ("ubi: Expose the bitrot interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>