drivers: ethernet: lan865x: Enable all multicast MAC addr for IPv6

Up till now the configuration of LAN865x was NOT allowing any
multicast frames to be received.

However, those are required for correct handling of IPv6's Neighbor
Discovery [ND] protocol.

This patch - by setting all bits in hash enable register - allows
all muticast MAC addresses to be recived.

Of course - more granular approach to specify set of addresses to
be allowed would be better and will be added in the future.

Signed-off-by: Stefan Bigler <linux@bigler.io>
[Cleanup + modified comment]
Signed-off-by: Lukasz Majewski <lukma@denx.de>