net: dsa: mv88e6xxx: Set PTP Ethertype

For the 6352 and newer switches, the PTP Ethertype defaults to
ETH_P_1588. Hence it was not explicitly set. The 6165 however defaults
to 0. So explicitly set the EtherType.

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