net: dsa: mv88e6xxx: skip unused ports

The unused ports are currently configured in normal mode. This does not
prevent the switch from being functional, but it is unnecessary. Skip
unused ports.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>