mlxsw: spectrum_switchdev: Check notification relevance based on upper device

VxLAN FDB updates are sent with the VxLAN device which is not our upper
and will therefore be ignored by current code.

Solve this by checking whether the upper device (bridge) is our upper.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>