b53: allow ports with higher numbers than CPU port

Our code was assuming CPU port uses the highest number. My BCM53573
device has eth0 connected to port 8 and eth1 connected to port 5. While
working on support for it I tried to:
1) Enable all ports (including port 8)
2) Set CPU port to 5

I noticed port 8 is not accessible anymore. It was just a development
process but it seems like something worth fixing anyway.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Jonas Gorski <jonas.gorski@gmail.com>