net: dsa: fix incorrect function pointer check for MRP ring roles

The cross-chip notifier boilerplate code meant to check the presence of
ds->ops->port_mrp_add_ring_role before calling it, but checked
ds->ops->port_mrp_add instead, before calling
ds->ops->port_mrp_add_ring_role.

Therefore, a driver which implements one operation but not the other
would trigger a NULL pointer dereference.

There isn't any such driver in DSA yet, so there is no reason to
backport the change. Issue found through code inspection.

Cc: Horatiu Vultur <horatiu.vultur@microchip.com>
Fixes: c595c433 ("net: dsa: add MRP support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/switch.c

@@ -729,7 +729,7 @@ static int
 dsa_switch_mrp_add_ring_role(struct dsa_switch *ds,
 			     struct dsa_notifier_mrp_ring_role_info *info)
 {
-	if (!ds->ops->port_mrp_add)
+	if (!ds->ops->port_mrp_add_ring_role)
 		return -EOPNOTSUPP;
 
 	if (ds->index == info->sw_index)
@@ -743,7 +743,7 @@ static int
 dsa_switch_mrp_del_ring_role(struct dsa_switch *ds,
 			     struct dsa_notifier_mrp_ring_role_info *info)
 {
-	if (!ds->ops->port_mrp_del)
+	if (!ds->ops->port_mrp_del_ring_role)
 		return -EOPNOTSUPP;
 
 	if (ds->index == info->sw_index)