net: dsa: make the STP state function return void

The DSA layer doesn't care about the return code of the port_stp_update
routine, so make it void in the layer and the DSA drivers.

Replace the useless dsa_slave_stp_update function with a
dsa_slave_stp_state function used to reply to the switchdev
SWITCHDEV_ATTR_ID_PORT_STP_STATE attribute.

In the meantime, rename port_stp_update to port_stp_state_set to
explicit the state change.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/dsa/mv88e6xxx.h b/drivers/net/dsa/mv88e6xxx.h
index 26a424a..4944855 100644
--- a/drivers/net/dsa/mv88e6xxx.h
+++ b/drivers/net/dsa/mv88e6xxx.h
@@ -497,7 +497,7 @@
 int mv88e6xxx_port_bridge_join(struct dsa_switch *ds, int port,
 			       struct net_device *bridge);
 void mv88e6xxx_port_bridge_leave(struct dsa_switch *ds, int port);
-int mv88e6xxx_port_stp_update(struct dsa_switch *ds, int port, u8 state);
+void mv88e6xxx_port_stp_state_set(struct dsa_switch *ds, int port, u8 state);
 int mv88e6xxx_port_vlan_filtering(struct dsa_switch *ds, int port,
 				  bool vlan_filtering);
 int mv88e6xxx_port_vlan_prepare(struct dsa_switch *ds, int port,