net: if: Leave all joined IPv6 multicast groups at net_if_down

When network interface is taken down, leave only those
IPv6 multicast groups that were joined.

Change-Id: I414556c093ba67be7c13e6c86e0451465c2203f3
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>