net: dsa: felix: Fix CPU port assignment when not last port

On the NXP LS1028A, there are 2 Ethernet links between the Felix switch
and the ENETC:
- eno2 <-> swp4, at 2.5G
- eno3 <-> swp5, at 1G

Only one of the above Ethernet port pairs can act as a DSA link for
tagging.

When adding initial support for the driver, it was tested only on the 1G
eno3 <-> swp5 interface, due to the necessity of using PHYLIB initially
(which treats fixed-link interfaces as emulated C22 PHYs, so it doesn't
support fixed-link speeds higher than 1G).

After making PHYLINK work, it appears that swp4 still can't act as CPU
port. So it looks like ocelot_set_cpu_port was being called for swp4,
but then it was called again for swp5, overwriting the CPU port assigned
in the DT.

It appears that when you call dsa_upstream_port for a port that is not
defined in the device tree (such as swp5 when using swp4 as CPU port),
its dp->cpu_dp pointer is not initialized by dsa_tree_setup_default_cpu,
and this trips up the following condition in dsa_upstream_port:

	if (!cpu_dp)
		return port;

So the moral of the story is: don't call dsa_upstream_port for a port
that is not defined in the device tree, and therefore its dsa_port
structure is not completely initialized (ds->num_ports is still 6).

Fixes: 56051948

 ("net: dsa: ocelot: add driver for Felix switch family")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>