mlxsw: spectrum: handle NETIF_F_HW_TC changes correctly

Currently, whenever the NETIF_F_HW_TC feature changes, we silently
always allow it, but we actually do not disable the flows in HW
on disable. That breaks user's expectations. So just forbid
the feature disable in case there are any filters offloaded.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>