stmmac: Correctly report PTP capabilities.

priv->hwts_*_en indicate if timestamping is enabled/disabled at run
time. But  priv->dma_cap.time_stamp  and priv->dma_cap.atime_stamp
indicates HW is support for PTPv1/PTPv2.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>