We need a better way to signal this, perhaps in phylink_validate, but
for now just print this error message as guidance for other people
looking at this driver's code while trying to rework PHYLINK.

Cc: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>