ptp: Add generic PTP is_sync() function

PHY drivers such as micrel or dp83640 need to analyze whether a given
skb is a PTP sync message for one step functionality.

In order to avoid code duplication introduce a generic function and
move it to ptp classify.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>