ptp: Add generic ptp message type function

The message type is located at different offsets within the ptp header depending
on the ptp version (v1 or v2). Therefore, drivers which also deal with ptp v1
have some code for it.

Extract this into a helper function for drivers to be used.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>