If @port is unused, then dsa_upstream_port(ds, port) returns @port,
which means we cannot assume the CPU port can be retrieved this way.

The sja1105 switches support a single CPU port, so just iterate over the
switch ports and stop at the first CPU port we see.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>