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>