net: dsa: fix older DSA drivers using phylink

Older DSA drivers that do not provide an dsa_ops adjust_link method end
up using phylink. Unfortunately, a recent phylink change that requires
its supported_interfaces bitmap to be filled breaks these drivers
because the bitmap remains empty.

Rather than fixing each driver individually, fix it in the core code so
we have a sensible set of defaults.
Reported-by: Sergei Antonov <saproj@gmail.com>
Fixes: de5c9bf4 ("net: phylink: require supported_interfaces to be filled")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com> # dsa_loop
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/E1qOflM-001AEz-D3@rmk-PC.armlinux.org.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/dsa/port.c

@@ -1727,8 +1727,15 @@ int dsa_port_phylink_create(struct dsa_port *dp)
 	    ds->ops->phylink_mac_an_restart)
 		dp->pl_config.legacy_pre_march2020 = true;
 
-	if (ds->ops->phylink_get_caps)
+	if (ds->ops->phylink_get_caps) {
 		ds->ops->phylink_get_caps(ds, dp->index, &dp->pl_config);
+	} else {
+		/* For legacy drivers */
+		__set_bit(PHY_INTERFACE_MODE_INTERNAL,
+			  dp->pl_config.supported_interfaces);
+		__set_bit(PHY_INTERFACE_MODE_GMII,
+			  dp->pl_config.supported_interfaces);
+	}
 
 	pl = phylink_create(&dp->pl_config, of_fwnode_handle(dp->dn),
 			    mode, &dsa_port_phylink_mac_ops);