net: dsa: felix: implement port flushing on .phylink_mac_link_down

There are several issues which may be seen when the link goes down while
forwarding traffic, all of which can be attributed to the fact that the
port flushing procedure from the reference manual was not closely
followed.

With flow control enabled on both the ingress port and the egress port,
it may happen when a link goes down that Ethernet packets are in flight.
In flow control mode, frames are held back and not dropped. When there
is enough traffic in flight (example: iperf3 TCP), then the ingress port
might enter congestion and never exit that state. This is a problem,
because it is the egress port's link that went down, and that has caused
the inability of the ingress port to send packets to any other port.
This is solved by flushing the egress port's queues when it goes down.

There is also a problem when performing stream splitting for
IEEE 802.1CB traffic (not yet upstream, but a sort of multicast,
basically). There, if one port from the destination ports mask goes
down, splitting the stream towards the other destinations will no longer
be performed. This can be traced down to this line:

	ocelot_port_writel(ocelot_port, 0, DEV_MAC_ENA_CFG);

which should have been instead, as per the reference manual:

	ocelot_port_rmwl(ocelot_port, 0, DEV_MAC_ENA_CFG_RX_ENA,
			 DEV_MAC_ENA_CFG);

Basically only DEV_MAC_ENA_CFG_RX_ENA should be disabled, but not
DEV_MAC_ENA_CFG_TX_ENA - I don't have further insight into why that is
the case, but apparently multicasting to several ports will cause issues
if at least one of them doesn't have DEV_MAC_ENA_CFG_TX_ENA set.

I am not sure what the state of the Ocelot VSC7514 driver is, but
probably not as bad as Felix/Seville, since VSC7514 uses phylib and has
the following in ocelot_adjust_link:

	if (!phydev->link)
		return;

therefore the port is not really put down when the link is lost, unlike
the DSA drivers which use .phylink_mac_link_down for that.

Nonetheless, I put ocelot_port_flush() in the common ocelot.c because it
needs to access some registers from drivers/net/ethernet/mscc/ocelot_rew.h
which are not exported in include/soc/mscc/ and a bugfix patch should
probably not move headers around.

Fixes: bdeced75 ("net: dsa: felix: Add PCS operations for PHYLINK")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>