net: if: Change the addr param type in net_if_ipv6_maddr_add/rm

Make the IP address parameter const because we are not
modifying the IP address in net_if_ipv6_maddr_add() or
net_if_ipv6_maddr_rm()

Change-Id: I98c19de132e58c386f661e8a76a349d562a82c71
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>