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.c b/drivers/net/dsa/mv88e6xxx.c
index 0dda281..53c545c 100644
--- a/drivers/net/dsa/mv88e6xxx.c
+++ b/drivers/net/dsa/mv88e6xxx.c
@@ -1193,7 +1193,7 @@
 	return _mv88e6xxx_reg_write(ds, REG_PORT(port), PORT_BASE_VLAN, reg);
 }
 
-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)
 {
 	struct mv88e6xxx_priv_state *ps = ds_to_priv(ds);
 	int stp_state;
@@ -1215,14 +1215,12 @@
 		break;
 	}
 
-	/* mv88e6xxx_port_stp_update may be called with softirqs disabled,
+	/* mv88e6xxx_port_stp_state_set may be called with softirqs disabled,
 	 * so we can not update the port state directly but need to schedule it.
 	 */
 	ps->ports[port].state = stp_state;
 	set_bit(port, ps->port_state_update_mask);
 	schedule_work(&ps->bridge_work);
-
-	return 0;
 }
 
 static int _mv88e6xxx_port_pvid(struct dsa_switch *ds, int port, u16 *new,