net: ip: mld: Ensure MLD APIs work with offloaded interfaces

MLD APIs are commonly used across the codebase to configure IPv6
multicast addresses on network interfaces. Sending MLD reports however
works only for native interfaces as it uses low-level APIs. Therefore,
in order to make the APIs at least semi-functional for offloaded
interfaces as well (i.e. allow to configure multicast address on
the interface), return early in case interface is offloaded.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>