net: lan966x: Extend lan966x_foreign_bridging_check

Extend lan966x_foreign_bridging_check to check also if the upper
interface is a lag device. Don't allow a lan966x port to be part of a
lag if it has foreign interfaces.

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