[IPV6]: Remove unneeded pointer idev from addrconf_cleanup().

This trivial patch removes the unneeded pointer idev returned from
__in6_dev_get(), which is never used. The check for NULL can be simply
done by if (__in6_dev_get(dev) == NULL).

Signed-off-by: Micah Gruber <micah.gruber@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>