net: dsa: lantiq_gswip: fix and improve the unsupported interface error

While trying to use the lantiq_gswip driver on one of my boards I made
a mistake when specifying the phy-mode (because the out-of-tree driver
wants phy-mode "gmii" or "mii" for the internal PHYs). In this case the
following error is printed multiple times:
  Unsupported interface: 3

While it gives at least a hint at what may be wrong it is not very user
friendly. Print the human readable phy-mode and also which port is
configured incorrectly (this hardware supports ports 0..6) to improve
the cases where someone made a mistake.

Fixes: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/lantiq_gswip.c

@@ -1452,7 +1452,8 @@ static void gswip_phylink_validate(struct dsa_switch *ds, int port,
 
 unsupported:
 	bitmap_zero(supported, __ETHTOOL_LINK_MODE_MASK_NBITS);
-	dev_err(ds->dev, "Unsupported interface: %d\n", state->interface);
+	dev_err(ds->dev, "Unsupported interface '%s' for port %d\n",
+		phy_modes(state->interface), port);
 	return;
 }