net: dsa: mv88e6xxx: Set STP disable state in port_disable

When requested to disable a port, set the port STP state to disabled.
This fully disables the port and should save some power.

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