net: dsa: support use of phylink_generic_validate()

Support the use of phylink_generic_validate() when there is no
phylink_validate method given in the DSA switch operations and
mac_capabilities have been set in the phylink_config structure by the
DSA switch driver.

This gives DSA switch drivers the option to use this if they provide
the supported_interfaces and mac_capabilities, while still giving them
an option to override the default implementation if necessary.
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/port.c

@@ -981,8 +981,11 @@ static void dsa_port_phylink_validate(struct phylink_config *config,
 	struct dsa_port *dp = container_of(config, struct dsa_port, pl_config);
 	struct dsa_switch *ds = dp->ds;
 
-	if (!ds->ops->phylink_validate)
+	if (!ds->ops->phylink_validate) {
+		if (config->mac_capabilities)
+			phylink_generic_validate(config, supported, state);
 		return;
+	}
 
 	ds->ops->phylink_validate(ds, dp->index, supported, state);
 }