net: if: check for multicast address already registered

Add a check to stop a multicast address to be registered multiple times.
This can happen if the application is using net_if_ipv6_maddr_add()
directly.

Tested on the existing bluetooth/ipsp sample:

<wrn> net_if: Multicast address ff02::1 is is already registered.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>