net: if: Change the iface param in net_if_ipv6_maddr_lookup

Change the handling of iface parameter in net_if_ipv6_maddr_lookup()
function:
  * If the *iface is set to NULL, then return the found
    interface to the caller.
  * If the *iface is not NULL, then use that interface
    when doing the lookup.

Change-Id: Ia1f0365170ea9f3e615d189231160614a80d241a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>