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.uk


Signed-off-by: Paolo Abeni <pabeni@redhat.com>