net: ipv6: also allow token to be set when device not ready

When we set the iftoken in inet6_set_iftoken(), we return -EINVAL
when the device does not have flag IF_READY. This is however not
necessary and rather an artificial usability barrier, since we
simply can set the token despite that, and in case the device is
ready, we just send out our rs, otherwise ifup et al. will do
this for us anyway.

Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>