[IPV6]: Do not set IF_READY if device is down

Now that we add the IPv6 device at registration time we don't need
to set IF_READY in ipv6_add_dev anymore because we will always get
a NETDEV_UP event later on should the device ever become ready.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>