net: ptp: Add stub for ptp_classify_raw()

When NET_PTP_CLASSIFY is disabled, a stub function is required in
order that the drivers compile.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>