Setup the TXBD to enable TX timestamp if requested.  At TX packet DMA
completion, if we requested TX timestamp on that packet, we defer to
.do_aux_work() to obtain the TX timestamp from the firmware before we
free the TX SKB.

v2: Use .do_aux_work() to get the TX timestamp from firmware.
Reviewed-by: NEdwin Peer <edwin.peer@broadcom.com>
Signed-off-by: NPavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: NMichael Chan <michael.chan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>