net: lan966x: Split lan966x_fdb_event_work

Split the function lan966x_fdb_event_work. One case for when the
orig_dev is a bridge and one case when orig_dev is lan966x port.
This is preparation for lag support. There is no functional change.

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