src: Implement LWIP's IGMP MAC filter callback.

[jimmo]

This is used to register the multicast group address on the wifi
interface's filter, allowing frames to that group to be received.

This only applies to v4. See #24 for the equivalent v6 change.

Fixes micropython/micropython#9105

[jimmo]

Note:

• Unsure if the cyw43_delay_ms(50); is needed at the end of cyw43_ll_wifi_update_multicast_filter.
• LWIP manages the group memberships, so in theory cyw43_ll_wifi_update_multicast_filter doesn't need to validate if a group is already registered. It's not much code though to ensure that they don't get out of sync.

[felixdoerre]

this change and #24 have quite a bit in common. I've split my changes into two commits, and I think 2baa5c5 can be used directly here, I think we should agree on a common interface to set the mac addresses, so mcast registered for v4 does not conflict with addresses registered for v6. I left out the 50ms delay and it worked fine on my board, but that might be something that someone more familiar with the chip can decide. You also removed the hardcoded mcast-address for mdns. I am not sure if that is needed, but I imagine it was in there for a reason.

[jimmo]

You also removed the hardcoded mcast-address for mdns. I am not sure if that is needed, but I imagine it was in there for a reason.

LWIP invokes the callback with the mdns group address when the mdns hostname is configured. The existing hard coded address was a workaround for not implementing this callback.

I think 2baa5c5 can be used directly here, I think we should agree on a common interface to set the mac addresses, so mcast registered for v4 does not conflict with addresses registered for v6.

This PR should replace that commit. It provides exactly the same functionality just without needing the extra 60 bytes of RAM to manage the list.

And to my first note above, using the same method for v6 is a good reason to keep the code to track add duplicate detection in cyw43_ll, as I think LWIP can't do this across v4&v6.

[felixdoerre]

sounds good, I'll rebase the v6 PR on this one.

[peterharperuk]

This change is now in #24

[dpgeorge]

Thanks, this is a good improvement, definitely needed!

[dpgeorge]

this was originally cyw32; fixed during merged