mlxsw: spectrum: Disable lag port TX before removing it

Make sure that lag port TX is disabled before mlxsw_sp_port_lag_leave()
is called and prevent from possible EMAD error.

Fixes: 0d65fc13 ("mlxsw: spectrum: Implement LAG port join/leave")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>