net: dsa: mv88e6xxx: do not change STP state on port disabling

When disabling a port, that is not for the driver to decide what to
do with the STP state. This is already handled by the DSA layer.

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