net: dsa: felix: break at first CPU port during init and teardown

The NXP LS1028A switch has two Ethernet ports towards the CPU, but only
one of them is capable of acting as an NPI port at a time (inject and
extract packets using DSA tags).

However, using the alternative ocelot-8021q tagging protocol, it should
be possible to use both CPU ports symmetrically, but for that we need to
mark both ports in the device tree as DSA masters.

In the process of doing that, it can be seen that traffic to/from the
network stack gets broken, and this is because the Felix driver iterates
through all DSA CPU ports and configures them as NPI ports. But since
there can only be a single NPI port, we effectively end up in a
situation where DSA thinks the default CPU port is the first one, but
the hardware port configured to be an NPI is the last one.

I would like to treat this as a bug, because if the updated device trees
are going to start circulating, it would be really good for existing
kernels to support them, too.

Fixes: adb3dccf ("net: dsa: felix: convert to the new .change_tag_protocol DSA API")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>