net: stmmac: add separate warning for PTP not being supported by HW

Chips like Amlogic S905GXBB are supported by this driver but don't
have support for PTP. Add a separate warning for missing HW support
to differentiate it from other actual failures.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>