net: dsa: check tx timestamp request in core driver

Check tx timestamp request in core driver at very beginning of
dsa_skb_tx_timestamp(), so that most skbs not requiring tx
timestamp just return. And drop such checking in device drivers.

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