net: ena: Relocate skb_tx_timestamp() to improve time stamping accuracy

Move skb_tx_timestamp() closer to the actual time the driver sends the
packets to the device.

Signed-off-by: Osama Abboud <osamaabb@amazon.com>
Signed-off-by: David Arinzon <darinzon@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>