s390/qeth: add SW timestamping support for IQD devices

This adds support for SOF_TIMESTAMPING_TX_SOFTWARE.
No support for non-IQD devices, since they orphan the skb in their xmit
path.

To play nice with TX bulking, set the timestamp when the buffer that
contains the skb(s) is actually flushed out to HW.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>