net: lan966x: Extend FDB to support also lag

Offload FDB entries when the original device is a lag interface. Because
all the ports under the lag have the same chip id, which is the chip id
of first port, then add the entries only for the first port.

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