nfp: TX time stamp packets before HW doorbell is rung

TX completion may happen any time after HW queue was kicked.
We can't access the skb afterwards.  Move the time stamping
before ringing the doorbell.

Fixes: 4c352362 ("net: add driver for Netronome NFP4000/NFP6000 NIC VFs")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>