net: lan966x: Update mdb when enabling/disabling mcast_snooping

When the multicast snooping is disabled, the mdb entries should be
removed from the HW, but they still need to be kept in memory for when
the mcast_snooping will be enabled again.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>