The KSZ9477 allows HSR in-HW offloading for any of two selected ports.
This patch adds check if one tries to use more than two ports with
HSR offloading enabled.

The problem is with RedBox configuration (HSR-SAN) - when configuring:
ip link add name hsr0 type hsr slave1 lan1 slave2 lan2 interlink lan3 \
	supervision 45 version 1

The lan1 (port0) and lan2 (port1) are correctly configured as ports, which
can use HSR offloading on ksz9477.

However, when we do already have two bits set in hsr_ports, we need to
return (-ENOTSUPP), so the interlink port (lan3) would be used with
SW based HSR RedBox support.

Otherwise, I do see some strange network behavior, as some HSR frames are
visible on non-HSR network and vice versa.

This causes the switch connected to interlink port (lan3) to drop frames
and no communication is possible.

Moreover, conceptually - the interlink (i.e. HSR-SAN port - lan3/port2)
shall be only supported in software as it is also possible to use ksz9477
with only SW based HSR (i.e. port0/1 -> hsr0 with offloading, port2 ->
HSR-SAN/interlink, port4/5 -> hsr1 with SW based HSR).

Fixes: 5055cccf ("net: hsr: Provide RedBox support (HSR-SAN)")
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>