mlxsw: spectrum: Handle port leaving LAG while bridged

It is possible for a user to remove a port from a LAG device, while the
LAG device or VLAN devices on top of it are bridged. In these cases,
bridge's teardown sequence is never issued, so we need to take care of
it ourselves.

When LAG's unlinking event is received by port netdev:

1) Traverse its vPorts list and make those member in a bridge leave it.
   They will be deleted later by LAG code.

2) Make the port netdev itself leave its bridge if member in one.

Fixes: 0d65fc13

 ("mlxsw: spectrum: Implement LAG port join/leave")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>