net: if: Extend the usage of rejoining the multicast groups

It may happen that the interface is up but not yet running
when we issue the rejoin_ipv6_mcast_groups(). This can be
fixed by calling this function again right after the iface
is set to 'running' state in the notify_iface_up handler.

Signed-off-by: Marcin Kajor <marcin.kajor@nordicsemi.no>