net: dsa: b53: Fix CPU port for 58xx devices

The 58xx devices (Northstar Plus) do actually have their CPU port wired
at port 8, it was unfortunately set to port 5 (B53_CPU_PORT_25) which is
incorrect, since that is the second possible management port.

Fixes: 991a36bb ("net: dsa: b53: Add support for BCM585xx/586xx/88312 integrated switch")
Reported-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>