net: dsa: mv88e6xxx: order ptp structs numerically ascending

As it is done for all the other structs within this driver.

Signed-off-by: Hubert Feurstein <h.feurstein@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>