net: dsa: bcm_sf2: Also configure Port 5 for 2Gb/sec on 7278

Either port 5 or port 8 can be used on a 7278 device, make sure that
port 5 also gets configured properly for 2Gb/sec in that case.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>