Merge branch 'ptp-is_sync'

Kurt Kanzenbach says:

====================
ptp: Add generic is_sync() function

as multiple PHY drivers such as micrel or TI dp83640 need to inspect whether a
given skb represents a PTP Sync message, provide a generic function for it. This
avoids code duplication and can be reused by future PHY IEEE 1588 implementations.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>