net: phylink: add debug print for empty posssible_interfaces

Add a debugging print in phylink_validate_phy() when we detect that the
PHY has not supplied a possible_interfaces bitmap.
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/E1s15rq-00AHye-22@rmk-PC.armlinux.org.ukSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/phylink.c

@@ -1838,6 +1838,9 @@ static int phylink_validate_phy(struct phylink *pl, struct phy_device *phy,
 					     interfaces);
 	}
 
+	phylink_dbg(pl, "PHY %s doesn't supply possible interfaces\n",
+		    phydev_name(phy));
+
 	/* Check whether we would use rate matching for the proposed interface
 	 * mode.
 	 */