net: lan966x: Implement the callback SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED

The callback allows to enable/disable multicast snooping.
When the snooping is enabled, all IGMP and MLD frames are redirected to
the CPU, therefore make sure not to set the skb flag 'offload_fwd_mark'.
The HW will not flood multicast ipv4/ipv6 data frames.
When the snooping is disabled, the HW will flood IGMP, MLD and multicast
ipv4/ipv6 frames according to the mcast_flood flag.

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