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>