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

This reverts commit 7458bd54 ("net: dsa:
bcm_sf2: Also configure Port 5 for 2Gb/sec on 7278") as it causes
advanced congestion buffering issues with 7278 switch devices when using
their internal Giabit PHY. While this is being debugged, continue with
conservative defaults that work and do not cause packet loss.

Fixes: 7458bd54

 ("net: dsa: bcm_sf2: Also configure Port 5 for 2Gb/sec on 7278")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>