mlxsw: spectrum_switchdev: Fix mrouter flag update

Update the value of the mrouter flag in struct mlxsw_sp_bridge_port when
it is being changed.

Fixes: c57529e1 ("mlxsw: spectrum: Replace vPorts with Port-VLAN")
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>