net: stmmac: Silence PTP init errors on hw without PTP

Logging a PTP error on hw which simply does not support PTP is not very
useful. Moreover this message gets logged on every if-up, and if there is
no cable inserted NetworkManager will re-try the ifup every 50 seconds.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>