net: mscc: ocelot: fix race condition with TX timestamping

The TX-timestampable skb is added late to the ocelot_port->tx_skbs. It
is in a race with the TX timestamp IRQ, which checks that queue trying
to match the timestamp with the skb by the ts_id. The skb should be
added to the queue before the IRQ can fire.

Fixes: 4e3b0468 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>